Take custom screenshots.
Constrictor is a screen snapshot utility. It provides you with a moveable, resizeable frame that floats over all your other windows.
Snapshots can be copied to the clipboard or saved in a wide variety of image formats. Your saved snapshots can automatically have thumbnail icons added to them, and Constrictor can even tag them as screenshots so they can be easily found with Spotlight later. Snapshots can be triggered by a context menu, configurable hot keys, or even AppleScript!
Shareware for Mac OS X 10.4 and up.