Dwarf Fortress is a really good free PC game, has a huge amount of depth and is being developed by only one dude. Sure, it's learning curve is horrible and the graphics are ascii (you can get a tileset) but if you get into it, the game is awesome. Also seconding Cave Story, that game is amazing.
Some survey sites are legit, but it takes forever to actually earn anything. Sites that have you sign up for sweepstakes and credit card offers and so forth are probably less legit, all in all I wouldn't bother with that stuff.