Slugging is a self-organized carpooling system in the Washington D.C. area that developed in the early 70s.
The system of slugging is quite simple. A car needing additional passengers to meet the required 3-person high occupancy vehicle (HOV) minimum pulls up to one of the known slug lines. The driver usually positions the car so that the slugs are on the passenger side. The driver either displays a sign with the destination or simply lowers the passenger window, to call out the destination, such as "Pentagon," "L'Enfant Plaza," or "14th & New York." The slugs first in line for that particular destination then hop into the car, normally confirming the destination, and off they go.
No money changes hands and an implicit rule of silence is followed, unless conversation is initiated by the driver. (thx, askedrelic)
Update: This practice is also called casual carpooling in the Bay Area. (thx, adam)
Update: Michael Sippey shares a serendipitous casual carpool story.