PickupPal, Ridester, 511 Rideshare, to name just three. What might set the new "carpool community" Zimride apart is a tie-in with Facebook (it's also a Facebook app) that will give users an easy way to find out a thing or two about their potential ride partners before hopping in the car with them. Through Facebook, users can check out pictures and see what hitchhikers/drivers are into and pick a ride based on compatibility (or looks, if you wanted).
Aside from Zimride, another new online rideshare application (whose name I forget right now) is available on the iPhone/iPod Touch. You know, just in case you want to ride with someone who shares your sense of Mac style.