Try Dragon's Nest. It's an action RPG that's free to play on Steam. It has really cool combo-ing elements, a lot of customization, and I really like the dungeon layout. I recommend you give it a shot, if only for a while.

Airmech is a good free-to-play real time strategy game that is a mix between classic mech games and Transformers. You take your airmech (mech that can turn into a flying vehicle) and make units to destroy your enemy's base, taking over side bases in the process. There are player versus player, cooperative, survival, and capture-the-flag gameplay modes available.

Need for Speed World is an MMO based on the popular racing series. The gameplay is basically the same as the other games, but it includes more social aspects as well. NFSW combines the worlds of Carbon and Most Wanted, with a bit of new area in between. You can get pretty good cars without paying real money, but it takes quite a few hours (think about 17). 

I think Pac man and Bloons Tower Defence are good games, I guess. And how about pinball, that is very good as well!!

That is 3, I think I forgot to look at the list, I forgot.

How about http://archbattle.com/? A good free MMO, that doesn't allow players to buy anything to help them in the game (at least, that's how it was back when I last played it, but that was a couple of years ago). It's a simple strategy/management game, and it also runs on Android/iPhone.

One of my favorite games is tankionline
Tanki Online is a free browser online 3D-action game developed by AlternativaPlatform company.
The game combines the charm of Battle City from NES epoch, modern 3D-shooter dynamics as well as excitement of online games. With all these great elements Tanki Online doesn’t require HDD installation and can be run on any Internet-connected PC. Just complete the registration which won’t take more than a minute and take part in immense tank battles. The game is completely free. Each tank is controlled by a real person. Player vs player battles open great potential for fascinating challenges and cooperative actions.
http://tankionline.com/en/game/#friend=VhwK233hXZERT1tcIFx1UMCVZqOvHPPg2f4C4qQqrH5j2zeank26TdjL5O8egX8P

Warframe is always a fun game to play if you're bored

If you are up for a hilarious, bloody free browser game try out Happy Wheels!
http://www.totaljerkface.com/happy_wheels.php

Or perhaps you'd rather attempt to make this horse gallop! play CLOP!
http://www.foddy.net/CLOP.html

One good free game that I properly enjoyed found on the adult swim website(It's not THAT violent) is Westerado.

http://games.adultswim.com/westerado-action-online-game.html

It's basically a murder mystery western shoot-em-up where you have to find the murderer who killed your family by finding clues.(played entirely with the keyboard)

I found a remake of Counter Strike, it's name is CS-Portable.
http://www.cs-portable.com/

Annother game that I found combines Call of Duty with the power of Unity Engine. It's name is Rush Team.
http://www.asr-games.net/

It is a multiplayer shoot 'em up (MMOFPS) as well as CS-Portable.

Not sure if anyone has said Marvel Heroes. 

It's a nice point and click Diablo style game with Marvel characters. Pretty awesome. A little computer resource heavy. 

Galacta :

Monster MMORPG  

Like Pokémon, Monster MMORPG lets you collect, train and battle with Monsters. But better than Pokémon, it is an Online RPG game, with people cooperating and challenging each other all around the world! 
Some of the core features are fully working live PVP battles, NPC trainers, Arenas, Individual values, Training Points, 1,200+ Monsters, 520 game maps, very fluent game play, extreme cheat prevention system, market system that supports buy, sell, trade, game integrated chat system and a working private message system.

Emoteen134

Some games may already have been posted here, but this are the free games that I like to play. 

Need For Madness, free to play singleplayer racing game, also has a multiplayer function, but you need to pay about 11 dollars a year for that.
Still, the single player is very fun because you can create your own cars and stages.  

Stepmania, rhythm game, it is sort of an emulated version of dance dance revolution, can be very challenging and addicting. 

Osu!, also a rhythm game, I like it more than stepmania, because this one features an account system and online multiplayer, and you can track how many points you have scored and lots of other cool things.

OpenTTD, a open-source remake of transport tycoon deluxe, very fun and even features multiplayer with up to 255 players. 

Super Crate Box, a nice and challenging arcade style game, available on steam.

Team Fortress 2, Dota 2, League of Legends, I think almost everybody knows these free games. 

Finally, Trackmania Nations Forever, free to play racing game on steam, singleplayer and multiplayer, quite fun game.

I know it's not free but usually Humble Bundle features 4-6 really good games + an extra 2-4 if you go over the average price they charge.
The 4-6 games is pay as you'd like, you used to be able to pay 0$ not sure if that deal's still there but still 0.99$ for 4-6 games is really good and they're usually fun games available for Mac, Windows and Linux. Last bundle I bought had...

-Brutal Legends
-Trine 2
-LIMBO
-Bastion
-A Virus Named Tom
-Eets Munchies
-FEZ
-FTL : Faster Than Light
-Mark of the Ninja
-Rocketbirds : Hardboiled Chicken.

Another game, not sure if it's been mentioned or not but Combat Arms from Nexon is a really good Online FPS game with many modes to play, CTF, Zombies, Terrorist, etc. It's a bit like COD but better and free. The game also features hundreds of guns, a dozen melee weapons and a bunch of equip table gear to enhance your performance.The only catch is unless you have a program like Wine and know how to use it you won't be able to play on MAC (not sure for Linux).

I got two little known games in mind that not very many know about.

- Dark Space
Dark Space is an online space combat and strategy game where you choose from one of three factions (U.G.T.O., I.C.C., and K ' L u t h)
The universe is comprised of systems, each containing planets, most of which can have structures built on them when the player is flying an engineer or command ship.
Teams are constantly fighting over control of planets, and engineers can build on and repair planets to provide the team a safe haven when fighting on the front line.
The graphics for Dark Space are 3D, but all movement remains on a 2D plane, which lends to a very arcade-like game play.

- Shores of Hazer on (Mandatory spelling check hates that last word... put Hazer and on together.)
Imagine a game where you can design your own species, design your cities from the ground up, design your own spacecraft, and explore and colonize possibly thousands of star systems.
Sounds a bit like Spore, but here is where it gets good... S o H is an online game where you can interact with other players, nearly everything is done through the 1st person perspective, everything you build can be walked around in, even your space ships, so you and your friends can walk onto a space craft which you built, navigate the inside, and sit at a console to operate one of the several roles (helm, navigation, sensors, shields, engineering, power routing)

marigine war is a good game. you go against the computer or someone else.

Cave Story, lol. No, not the Plus version on Wii and Steam, it's just plain old Cave Story. I chose this one, because you can change weapons, and actually seem to enjoy the destruction of the rockets! Not just that, the music is pretty astounding!

Another one, though, is Spiral Knights, an MMORPG. You pretty much travel around, and fight monsters, like traditional MMORPGs. The thing is though, you don't level up in this one. You just get better equipment by exchanging your Crowns for Energy, the main currency. I guess this game isn't entirely free, because you have to buy Energy using real life money. The game is still free to play, though.

So...yeah, these are my two favorite F2P games.

Tyrian 2000 - Tyrian 2000 is a free, but awesome side scrolling space shooter (I don't think it follows the genre "bullet hell" since it does not spam you with bullets) back from the 486 days. The game requires you to get a GOG account for you to download the game, but the game comes pre configured for modern systems and you get a neat selection of games for you to buy.

System Shock - This game is technically not freeware but this new version called Portable is a free download. Sadly, still no official digital release of this amazing game. System Shock Portable is a special version of the CD version of System Shock that includes all of the new content of the CD version with higher resolutions, mods, and a pre configured setup for modern systems. Also, this game has a special bonus for you people who like to collect death scenes from video games.

Battlefield 1942 - I don't need to go in depth about this one. BF1942 is a legendary multiplayer FPS that is part of a now legendary series. Game requires a Origin account, but it is still worth it.

SimCity - I know what you are thinking. "But Fablefire, this game is named Micropolis, it can't be the same SimCity everybody knows and loves!". However it is indeed SimCity, just renamed to dodge copyright issues. (Just like what happened to the free game Ur-Quan Masters, which I have not played yet.) The game is Java based, so it is going to work on almost anything that is not DOS or Windows 1.0-2000.

BlackThorne - Blizzard has recently released one of their earliest games for free, with that game being BlackThorne. As always, this release is pre configured for modern systems. Like Flashback? This is the game for you. 

Sun-Wukong : dddddddddhddddddddddddddddddddddddddddddddddddddddddddddddddddddddd hghgg ghghg ghg gh ggh  ghgh gh ggh g hg h g hgh gh gg hg hg hg hgh ghg hggh g hih ih ih ih hih ih ihih ih h ih ih ih ih ih ih ih ih ih i hih ih  hihihiih i hih hi i i i i i i i ii ii ii i i i i ii  i i  ii i i i i i i i i i i i i i ii i i ii i ii i ii i i i  i i i i i i i i i hi broooo oo o o o oo o o o oo o o oo o o o oo o o o o o o oo o o o o o o oo o o o oo o o o o oo o o o o o o oo o o o o o o o oo o o o o  o o o o o o o o  o  oo o   o o  o o o o ooo o o o o  o o o o o o oo  oo o o o o o o o oo o o o o o o  oo o o o o o o o o o oo o o o  o o o o o oo o o oo o  oo o o o o oh yaaa free games to get... Nfs world is a great game to get if u love to racejfhfj  jffj jfjfjffjsjdfjsihg ujauiu dsiusfdisaduifadsu  uf uu ufu f uf ufdosiopisfpispo   opi po p p p [][ [ [ [ [ [ [ [ [  [  o iuj  i i iu iui iu ui u u uu u u uu u u u u u u u uuu u uu u u uu u u uu u u u u u uu u u u uu u uu u u u u u u u uu  uu u u u u u u u u u  u u u lkjdkjfsdj is  dis jksjfi jisd k choa fodjsif i dsfi iodi idioadsjf ajido jaioi s ifjididji dij jdfi idjf ifjd djji dji jiofdjsosa fjfjf f f f ff f f f f f f f f  f f f gg g g g h h  h jj k k l l l ll l ;; ; abcdefghijklmnopqrstuvwxyz now i now my abcs next time wont u sing with me. sorry for typing random words, i need to earn Viz so i can save up for the playstation thingy, not there yet but when i am, i cant wait kk 

If you want free games consider downloading virtual consoles and downloading ROMs to play. You can get most consoles, PS2, PS, Wii, Nintendo DS, Game boy Advanced, etc. I know Vizzed has a lot of games but they don't have everything and some of it usually costs Viz and requires you to have a Windows OS or a replacement like Parallels, Wine, etc. To use the Vizzed plug-in. You can even buy cheap controller adapters to be able to play console games online with console remotes like gamecube or N64. All programs using vitual machines also have save states so no worries about restarting each time.