Casual carpooling is the quite common in some areas. The one I'm most familiar with is between northern Virginia and DC. There are "slug" [0] lines scattered around, commonly at commuter parking lots. I-395 has HOV-3 lanes on it, and it can cut a commute from around between an hour to an hour and a half to around thirty minutes. Passengers line up, and as a driver pulls up, he tells the person in the front of the line where he's going and how many he needs. The message is passed down the line (typically by the head of line yelling it out), and passengers step out of line and get a free ride in.

[0] http://en.wikipedia.org/wiki/Slugging