Flickr junkies should be happy to find that the fine folks at Flickr have added a new feature called Collections, which allows users to categorize and organize their Flickr photo sets.
Actually, it looks like you can go pretty deep with the whole Collections idea - creating not only collections of sets, but also collections of collections (up to 5 collections deep) - meaning that if you've got a lot of sets, you can now set up a very organized folder structure-type collection of your photos. Also, sets can be placed into as many collections as you want, so the sky's the limit.
Flickr collections visually differentiate themselves from Flickr sets by using the mosaic-style icon you see in the screenshot above. Unfortunately it looks like collections are Flickr Pro-only, though any plain old Flickr account holder knows you don't get enough sets with you free account to have much of a need for collections anyway.