BuddyPress.org

As PCWriter said, if you’re collections are picture-based, BP-Albums could do the trick.

If your colelctions are more information-based, I don’t know of any plugins that would fit your needs right out of the box. Do you have a target “collection type” in mind? You mentioned paintings – would it only be paintings for your site?

I think it would be tough to make a generic “collections” plugin. The info you would want for baseball cards, paintings, bottle caps, books, etc are all somewhat different. If you have a client or a site with one specific collection, I would start there, and then see what you could change to make it more generic (if that’s your goal).

That being said, I think your idea sounds cool, and many people would dig such a plugin.