Some Web 2.0 concepts don't make sense to people unless you break it down to them in ways they can understand. We do our best with our Newbie Guides for things like Twitter, Flickr, Google Reader, and Facebook. Along similar lines comes a video about Del.icio.us, and social bookmarking in general, from Common Craft--a consulting company that does Web videos. This may be viral marketing, but it's very well executed and a joy to watch. I'd hire these guys for my start-up video.