In post-soviet countries like Poland it was (less so now, but still popular). At some point we even had "hitchhiker's booklets" with coupons you would leave drivers who took you, and they could then use in lotteries. Hitchhiking was a strong tradition in Central/Eastern Europe, and - in some parts of the continent - still is.

Apparently it is a thing in DC, known as "slug lines": http://en.wikipedia.org/wiki/Slugging

The idea is that by picking up people, you can drive in the HOV lanes.

It's common in San Francisco: http://sfcasualcarpool.com/