Constrictor is a shareware screen snapshot utility for Mac OS X. It lets you position a frame over exactly the area you want to snap, and then lets you customize the snap image with effects like framing and feathering. It has considerable flexibility in saving, and can take snaps using hotkeys without interrupting your workflow. And it’s scriptable, too!
Version 2.3 is for Mac OS X or earlier. Users with later versions of Mac OSX should download version 3.0