Macworld's Mac Gems blog had a snippet just recently about a program called "Cinch". Cinch brings the "auto resize" feature found in Windows 7 to the Mac.
Not sure if you've ever tried it, but I find it quite handy - particularly with a widescreen machine. Basically, you just drag a window to the far right of the screen until your pointer touches the border. The window snaps to take up exactly half of the screen. Drag a window to the extreme left, and it snaps to the left, making for an instant split screen. This is particularly handy if you need to compare the contents of two windows or want to monitor multiple processes, giving equal credence to both in terms of screen real estate.
To me, this is basically the best thing about Windows 7 and something I miss badly when I'm using my Mac. It's $7 and was well worth it for me. Just thought I'd share.
