Awesome Freeware games that will steal your life:
Dwarf Fortress - you control dwarves and build a fortress, and die (you may be confused at first, and not understand anything, but this graphics mod [http://mayday.w.staszic.waw.pl/df.php] and this wiki [http://dwarffortresswiki.net/index.php/Main_Page] will be your friends)
Nethack - You create a character and explore a huge dungeon, and die.
Infiniminer - You're supposed to find the most gold and be richer than the other team, but sandbox mode is more fun: build and explore (fun, but not enough players anymore, so kinda lacking)
Toribash - wierd fight sim (I never got into it, as I didn't really want to play online and get my ass handed to me constantly, but it was pretty fun)
Torus Trooper - you fly down a tube, shooting and dodgeing enemies (I dl'd this forever ago. I have no idea if it's changed since)
Strange Adventures in Infinite Space - you pilot a ship, and go to planets that each have stuff on them, and gather as much as you can (really short, but fairly replayable)
Clonk - You control these things that will build stuff for you and dig and I think I'm missing something about the game, yet it's still fun.
That's just what I got off the top of my head. I could list all the freeware shooters and MMO's on top of that, but they generally aren't as good. I'd also count flash games, but then the list would never end.
Dwarf Fortress - you control dwarves and build a fortress, and die (you may be confused at first, and not understand anything, but this graphics mod [http://mayday.w.staszic.waw.pl/df.php] and this wiki [http://dwarffortresswiki.net/index.php/Main_Page] will be your friends)
Nethack - You create a character and explore a huge dungeon, and die.
Infiniminer - You're supposed to find the most gold and be richer than the other team, but sandbox mode is more fun: build and explore (fun, but not enough players anymore, so kinda lacking)
Toribash - wierd fight sim (I never got into it, as I didn't really want to play online and get my ass handed to me constantly, but it was pretty fun)
Torus Trooper - you fly down a tube, shooting and dodgeing enemies (I dl'd this forever ago. I have no idea if it's changed since)
Strange Adventures in Infinite Space - you pilot a ship, and go to planets that each have stuff on them, and gather as much as you can (really short, but fairly replayable)
Clonk - You control these things that will build stuff for you and dig and I think I'm missing something about the game, yet it's still fun.
That's just what I got off the top of my head. I could list all the freeware shooters and MMO's on top of that, but they generally aren't as good. I'd also count flash games, but then the list would never end.