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Screenshot Captor: The most full-featured screenshot app I've seenScreenshot Captor is an advanced, full-featured screenshot application boasting an impressive feature set that rivals the shareware favorite, SnagIt. In all, Screenshot Captor may be the most full-featured screenshot app I've seen, freeware or otherwise. On the other hand, it does have a slightly steeper learning curve, so if there's a downside, that may be it.
MiniCap is a minimal screenshot capture app intended to be invoked from the commandline or bundled with and called from other tools.
MiniCap has 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.