Adium just keeps getting better and better. While messing around with one of Adium’s features the other day, I found a way to split up your contact list into multiple sections, by group.
To break up your contact list, hold Command and drag the header for any of your groups off of the window it is currently in.

You can put two windows of groups together either by holding Command and dragging them back in the same way, or simply dragging (without Command) one of the windows under the other one until it snaps into place, and then letting go. Another way is to press Command-W while the focus is on the window which you want to attach to the main window.
You can combine this trick with this one, which allows you to create a group for offline contacts, and then split those off to the side.
Another way to use this trick productively is to use it when combining contacts, which we discussed here. If you have a long list of contacts and/or are combining contacts from multiple groups, being able to split your contacts list up makes combining contacts much more manageable.
Like many of Adium’s features, this trick is very flexible and can be taken advantage of in more than one way, so try playing around with it a little bit and see what it can do.