A brilliantly simple and elegant way to detect which social networking or news sites an individual user visits. Then you could display the badges that they actually care about, rather than all the badges you can think of.
For example, imagine adding the StumbleUpon badge nice and big if the user’s a StumbleUpon user, but hiding it otherwise. Or only showing the Reddit badge if the visitor is a Reddit user, and displaying the more recognizable Digg badge otherwise. It’s a simple and elegant way to give the individual visitor what they’re most likely to use to share your content, and I’d imagine it’d be quite effective in an A/B test. (If we have a chance perhaps we’ll try it ourselves.)