I would say the Orange Box is a must. Easily the most-purchased thing on Steam, gives you access to Portal, Half-Life 2, and Team Fortress 2.
If FPS's aren't your cup of tea, I would shorten this to just Portal . Every self-respecting person even vaguely interested in videogames has to play it at least once.
Super Meat Boy is really popular right now, and I can confidently say that it is indeed quite fun.
Audiosurf is another indie game, and turns the music on your computer into levels for it. I thought it was nice, and since I'm kinda an audiophile I enjoy it.