The largest such system I'm aware of is slugging in the DC area (particularly along the 95/395 corridor to Pentagon/downtown): at various parking lots, a driver pulls up to a line of people, says they have 3 slots and is heading to the Pentagon (say), and 3 people pile in and get to ride the HOV to the Pentagon.

It's entirely free, almost entirely grassroots (governments occasionally ask that the pick-ups/drop-offs move slightly), and remarkably safe (1 known incident in >30 years). Having grown up in the area, I was surprised when I casually made reference to it to someone not in the area and they had no idea what I was talking about.