May I introduce you to Locomalito? He's a Spanish indie developer with an extremely low budget and 100% free games. While this is usually a recipe for disaster in the current indie game "industry", the guy is extremely dedicated to all of his works, which combine elements from multiple old-school games with a grain of history (both personal and worldwide) into beautiful masterpieces.

Genetos is another excellent free retro-inspired indie, this time by Japanese dev Tatsuya Koyama. It's a loving tribute to the history of shoot 'em ups/shmups/STGs, with five stages representing each step of the evolution of this genre that changed so much since it's creation, from a Space Invaders ripoff to a DodonPachi/Touhou-style bullet hell with tiny hitboxes, scoring systems, bombs and sub-weapons. While it is terribly short even for a shmup and it's MIDI songs sound somewhay cheap, it's strongest point is it's replayability, fueled by an interesting power-up/achievment system.

Also, the original version of La-Mulana. Anyone familiar with the current indie scene may be familiar with this name. Even though you have to pay for the remake available on Steam and WiiWare, the original version is completely free and very different from the recent re-imagining, being much closer to the brutal MSX/NES metroidvanias that inspired it. The way it's programmed makes it only run right on a PC with Windows XP or older and you'd have to download and install Aeon Genesis' fan-translation patch to fully understand it if you don't know Japanese, but it's a maginificent, brilliantly brutal game anyway and a good choice for those who doesn't have enough money for the enchanced version (like me ).

Try the ones at STEAM. Some of them are not bad if you have a Steam account.

One free game that's become popular recently, particularly on iOS, is called 2048. No, it's not based in the future, it's about powers of two. Basically, you have this screen with numbered tiles on it, starting with 2's and 4's. You slide up, down, left, or right to move all of the tiles in that direction, and if you collide two of the same kind, they form a single tile with the sum. Everything's based on powers of 2, so two 2 tiles become a 4, two 4 tiles become an 8, and so on. The "goal" is to reach the 2048 tile, but you can technically play beyond that, at least you can on the iOS app as of a recent update. The brilliant thing about it it that the progress is so slow, but it never actually feels slow, because the early steps (that is, colliding the 2 and 4 tiles) are so quick and easy, that progress is nearly constant as well. You lose when the board fills up and there's nothing to collide. Every move forms an extra 2 or 4 tile. It's like the new Flappy Bird, an incredibly simple, popular, and often cloned iPhone game. Difference is, unlike Flappy Bird, it doesn't suck. Plus, you can make your own version with custom numbers or pictures here.

Defence of the Ancient 2 commonly known as DotA2, in the game there is two sides The Dire and The Radiant, in both sides there are 5 heroes which are controlled by the players (every player controls 1 hero), then there is an NPC army that automatically tries to destroy the enemies Ancient, but before they can reach the Ancient they, you and your ally players need to destroy the defensive towers to reach the enemy base. while trying to kill the enemy players and try not to get killed... Very hard to explain here, because I suck explaining things. But if it sound interesting you should check it out.

Huh.. I never knew Team Fortress was a free game. I hear people talking about it all the time, but I've never played it myself. I guess I kind of figured it was a game you pay for, based off its popularity.

I used to play a game called cantr.net, if anyone likes role-playing games. It's free and totally awesome, but I refuse to play it thanks to the corrupt ownership and admin system.    If you have no reason to interact with the staff though, you'd likely love it.

Spiral Knights I love to play that game, though I admit Its a little hard to play if you don't intend to put real money in.

Phantasy Star Online 2 is free. Fun little MMO right there.

I am surprised nobody has mentioned the freeware version of Spelunky yet. It is an awesome rouge-like game. I'd explain it more, but just try it for yourself. You will be glued to your computer. 
Link: http://spelunkyworld.com/original.html

Also, there is this little MMO I found called Saturn Valley Online. Doesn't work on my computer but should on yours. It is a Earthbound MMO. 
Link: http://www.skyon.be/

FLATOUT 2 Is A Good Game Of Cars...                                                 You Can Play Races Derbies and Stunts.....                                                                                                                                                                                                                                                                                                                             There Are Many Cars To Choose From......

I would defiantly recommend warframe as its a nice space shooter with wall running physics gun play stealth or super commando its free on steam or on ps4 and its a well rounded game best part is you don't ever have to spend a dime if you know your stuff because of the way the game is made its not the type of f2p model typical of this generation its not pay too win its more like pay to get more stuff that you can store with or buy this funny hat to support the devs. Another good reason to try this game out is that the events are plot based and each event tells the next part of the story its even summarized in the codex menu under events for each one everyone missed. You even can get some nice new stuff and free gear as well that my friend is a great example of a good free to play game.

another good game I would recommend would be vindictus since its a super hack and slash completely destructible environment rpg with party elements you have a wide variety of stuff to choose from and a nice arsenal of stuff each character you pick. By character I mean the characters represent the classes and as of this post there will be another addition to the team out of the speed swordsman -technical- the speed swordswoman the archer the mage the sword and shield woman the great swordsman they each have their unique play style and own name associated with them I.e. karrok,lann etc etc. Pretty much the game revolves around class types so choose wisely. This game is currently on steam for download and runs on the valve source engine.
Both of these games are great free to plays for the not so pay to win aspect which I find very rewarding as I do not have to have money stolen from my wallet just to advance onwards to a new segment which a lot of these f2p games are plagued with.

I don't think anyone has mentioned this, but those of you who are interested in puzzle games, there is a free game made by Digipen Institute of Technology students in 2012 that people can download, even though it's one of their projects in class.

http://www.seewithperspective.com/

It's called Perspective, and you pass through different rooms and avoid destructive orange borders by freezing the frame and changing the perspective of the game for your character to move on. It's a pretty smart design and it's interesting to play through.

Aura kingdom is a great free game, it's an mmo it's pretty good and the graphics are very anime-y ish, but yeah check it out, if you're into those kind of games, http://aurakingdom.aeriagames.com/ or download it on steam

Not sure if it's been said but,

Armagetron Advanced! It is basically just like the light cycle scene from tron, but you play it!

http://armagetronad.org/

So fun!

Super Mario 63 (Flash Game)
A 2D version of Super Mario 64 with F.L.U.D.D. from Super Mario Sunshine, offers some puzzles, but is a full flash game. It's one of the best Flash Games all time, definitely worth the try, there are 64 Stars and 64 Star Coins.
You can also play as Luigi if you have 32 Star Coins!

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