From allSnap's home page:

allSnap is a small system tray app that makes all top level windows automatically align like they do in programs such as Winamp or Photoshop.

I have heard about it while listening to Security Now! Episode #65. They say:

...Ivan Heckman's wonderful and clean little Windows "allSnap" utility (which causes all Windows windows to snap to screen and other window borders)...

I have been using it for a few weeks now and I am very pleased with it. I can not really say much about such software, and why I find it useful, you will just have to try it. Leave a comment if you also find it useful (or not).