1. Super Mario Bros. Crossover
In this game you can choose between six characters from other classic NES games to play on the format of Super Mario Bros. Each of these characters can use their own powers from their original games to help them jump, fight, and throw their way to victory.
2. Realm of the Mad God
This mmorpg is one of the most popular games I know. Or at least that's what I've heard. No one I know seems to know about it but when I made an account there were millions of players online. Basically all you do is wander around the world and use the weapons and armor that you find to defeat the endless waves of enemies with your fellow gamers. It's a pretty fun game and it's easy to get lost in, that is, until you die and then you don't want to play for a while.
Spelunky is an open source Indiana Jones style game in which you control a spelunker who dives into caves to find treasure, save damsels, and dodge traps. There are two things I absolutely love about this game. One is that the levels are randomly generated so you will never undergo the same challenge again. The second reason is that there are many mods or modified versions of the game which are available mostly on the forums. While playing this you will die a lot and it'll be frustrating but you'll learn eventually!
4. Hobo Wars
Hobo Wars is a role playing game in which you join gangs, train, ride shopping carts, go on adventures, breed rats, and fight thousands of other players around the world. Not only is it creative but it really is as good as it sounds.
This game is a genius mix of Super Mario Bros. and Portal! Of all of these games this is the best I have played for free, ever. It's almost indescribable because it just is what it sounds like. Mario holding a portal gun, who would have thought? I won't describe it too much so you can hurry up and download it!