allSnap

If you're as messy as me, you probably have windows cluttered all over your screen. And if you're as anal as me, you probably hate to see even one pixel go to waste. allSnap is a useful little program that will make your windows "snap" together very easily.

The author's webpage mentions that it uses a techinque called "hooking" to make the windows snap, and that supposedly this is supposed to be a bad thing. I personally have been using allSnap for probably over a year, and I've never seen it cause any problems.