MiniCap

MiniCap is a minimal screenshot capture app, with good commandline support, including the ability to launch (and then terminate) apps, capture windows by handle, processid, process name, or interactively.

MiniCap has a convenient interactive capture mode (based on Screenshot Captor's Redbox Capture mode) which may be useful if you don't want to have something like Screenshot Captor running in the background. But where MiniCap really shines is it's use as a non-interactive commandline tool.