I'm not sure if I'd have time to develop this myself, but I thought I'd throw this idea out there to see what others think about it.

The idea would be to have a firefox extension that allowed users to quickly select a portion of their browser screen to save it as an image to their profile on the bookmarking service. This could be a great way to keep track of all the little aspects of websites you find interesting/inspiring (menu systems, color choice, features, etc). You could then go to your profile and browse all your images, or browse by website (so you could check out a collection of images you saved of a particular website).

Others could come to the website to browse all the UI images and even subscribe to users to keep up to date on what their bookmarking.

I find 37signals "screens around town" blog posts, where they share images and their thoughts on particular design decisions, to be really interesting. My idea would be to form a service / community around this UI sharing concept.