W3 Photo
The SemanticWeb, like WikiSoftware, seems to be a concept which instantly alienates or appeals to people. Having the web be inherently searchable and classifiable seems to defy imagination, yet it is one of those ideas where the more you think about it the cooler it becomes. Even though I can't think of a practical reason to annotate my photo collection, the concept appeals to my obsessive organizing nature.
I've talked about FotoNotes before, but now the W3 seems to be using something similar (possibly the same software that powers FotoNotes?) for old conference pictures and is asking the public to help annotate them.
You have to login before you can contribute, but if there is a region of the picture you know about that hasn't been described, you simply draw a box around it with your mouse and it will open a window asking you for a description. Here's an example of an annotated picture.
See also: http://fotonotes.net/, http://flickr.com/