goproblems.com

Another benefit of having a pro account could be the ability to create collections of problems. These could work basically like adding problems to the "favorites" list, but users could create their own categories or themes. www.chessgames.com has this feature if you wanted to see an example of what I mean. Thanks!

Well I just noticed that there is a similar feature through the "groups" option on the problem search page.

Please be aware that groups are shared by all the users. You can't use them for your personal purpose.

Tails,
The model I was thinking of was the chessgames.com model, and there the collections are open to others to view as well (here, if I understand properly, there is also the option to disallow others from modifying the collection). The main difference I think is that the collections there are accessible from personal pages rather than from a main page so they feel a bit more personal, but that's a minor point I think. Thanks.