So I bought those two games from Steam yesterday.

Audiosurf is so awesome, it makes you forget you even bought Guitar Hero/Rock Band.  The speed and the difficulty depends on the song you're using, and baiscally the songs you have in your hard drive/MP3 player/audio CDs/whatever you use are in-game.  That's right, Audiosurf can read WMAs, MP3s, OGGs, and audio CDs, along with Audiosurf Radio if you're too lazy to use your own music.

Left 4 Dead in terms of awesomeness is self-explanatory.  Don't bother playing single-player, although good, it's nowhere near as fun as playing with friends (or complete strangers LOL).  I never played versus mode yet, but it sure sounds fun to be the zombie.  I'm more interested in the co-op mode, as going through the four campaigns are more fun than doing it alone.  Personally, I love this game, truly redefines the zombie horror genre, although there's Resident Evil Outbreak (LOL) to look into.

Both are fun.  If you want to play with me or if you want me to kill you just because I'm in an angry mood in L4D, my Steam ID is dalejrgamer.  And yes, Pvt. Rod Stewart is my alternate name.

I got L4D for PC, if you haven't noticed already.