I was introduced to StumbleUpon by Matt with this wonderfully pointless piece of web art. Essentially, StumbleUpon is an extensions/add-on for your browser that adds functionality. In this particular instance it is a toolbar that enables you to surf web content that has been identified as of interest to you through software that “combines collaborative human opinions with machine learning of personal preference”.
The developers claim that it is a way of automating word-of-mouth referral by the development of online social networks of like interested people. This may or may not turn out to be the case but it is at least a relatively simple to install and good fun if nothing else.
This is an example from the bizarre/oddities category.