Get a Steam
account and go shopping.
If you like shooters, the big multiplayer game right now on PC is still Bad Company 2. If you liked Team Fortress, you could always give TF2 a chance, it's fun as hell and has a really cool style to it (plus it's only like 20 bucks).
Luckily there are a bunch of really great looking games coming out over the next little while so there's no shortage of cool shit to play.
Not sure what's great in terms of racing games, I don't play those on PC.
...what are your PC's specs? Do you have a good video card?