[Video Link] Divvy doesn't do anything except let you resize OS X and Windows windows by snapping them to widely-spaced gridpoints. I watched the above video, and was interested enough to download the demo (which doesn't expire but has nags about 2 or 3 times a day). I ended up liking it so much I bought it for $14.
I use it dozens of times a day. For example, I use it to set up a couple of Finder windows and a FileMaker window (so I can compare MAKE stories that are on my hard drive, on MAKE's file server, and in the editorial database.) I use it to quickly pop open and resize side-by-side Chrome browsers so I can write a blog entry. I find more uses for it almost every day.
Divvy offers configurable keyboard shortcuts – I use "1" to fill the left half of the screen and "2" to fill the right half.
This is something OS X and Windows should have baked into their operating systems.