Because I like games, and I don't have money.
Maybe if this actually becomes interesting/cool/useful, we could sticky it or... I dunno.
At any rate, I would think we are more or less in agreement that Egoboo, Cave Story, Wesnoth, and Nethack should be on there. Thus:
Egoboo
If you're here, and you don't know anything about this, well... that's strange. Download it now.
Doukutsu Monogatari (Cave Story)
Cave Story is a platform shooter and is sometimes described as a "metroidvania." It's more exploration-based and isn't really divided into stages. It has several different guns and upgrades, a unique weapon leveling system, and a neat story with interesting characters and setting.
Its look and sound are very "old school" and it looks something like a SNES game.
The Battle for Wesnoth
Wesnoth is a turn-based strategy game with a fantasy setting. It has online multiplayer with several different factions to play as, as well as several long campaigns to play through. You can also just play a single game against AI, or against other people using the same computer (hotseat).
Nethack
Nethack is a roguelike / dungeon crawling game. It is incredibly deep and there are a lot of strange secrets to the game. It's nearly impossible to actually beat the game, though, especially since there are about three billion ways to die.
And here some other games; I think they're amazing, but I don't think (m)any of you have played them, so try them out:
N
N is a fairly simple platformer: You control a ninja, and your goal is to collect gold, open the door to the exit, and get to it to finish the level. You are timed for each level, and there are various robots trying to kill you. It features very nice physics (it's actually quite entertaining to watch your character die) and many, many different levels.
Iji
Iji is another platform shooter-ish sort of game, and although there are a lot of secrets and sort of an exploration aspect, it is stage-based, and often times there's no way to go back to where you were before, so it encourages multiple plays through. It has an interesting story, and the game changes somewhat based on your choices (whether you choose to kill as little as possible or destroy everything in sight).
There are plenty of extra unlockable things and secrets to find, too.
Savage: The Battle for Newerth
Savage is an interesting online game and it's sort of hard to describe. Basically it's a combination of an FPS and an RTS; you can be the commander and play it like a real-time strategy game, or you can control one of the actual units on the field. It has a unique setting that is sort of bronze age / early iron age-ish and sort of steampunk and sort of I don't know what all else.
Apparently the sequel is also free now, but I have not played it because it won't run on my computer.
So yes. Recommend games here. But not just good ones; amazingly great games.