The Ron Paul campaign has tended to be the example here, but I'm fascinated by today's addition: YouBama. It's pretty much what it sounds like: a specialized YouTube variation, specifically for populace-generated Obama support material. The site claims, plausibly, to have nothing to do with the official campaign -- it's a couple of students playing with participatory democracy. But it's a fine and timely idea: an unofficial place where supporters can put their words and videos, and vote for which ones work best.
Neat stuff. I wonder how long before backers of the other candidates catch on? It's not surprising that Obama supporters did this first (indeed, the TechCrunch article that I got the pointer from reminds us they they're endorsing Obama precisely because he seems to be the most tech-enclued of the candidates), but it would be pretty easy to copy...