Flickr launched its collections feature today.

From the Flickr blog:

What are collections, you ask? A collection is a container into which you can place either sets or other collections, allowing you to create a hierarchy as deep as 5 collections. You can place as many of your sets into a collection as you like, and a set can be in as many different collections as you like.

You can also choose various layout options your pages to display sets, collections and specify the size of the images shown.