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Pro Free and open source
Greenshot is licensed under GPL with source code available on BitBucket.
Pro Can capture an entire webpage
Instead of just capturing the visible portion of the page, this tool can capture all of the page in one image by scrolling through the page, taking images at set points and then stitching the images together.
Pro Allows annotating, highlighting and obfuscating screenshots
Pro Freezes screen when you capture
When you enter a capture mode, the screen content is preserved as it was when you started capturing.
Pro Can be quickly invoked with the print screen button
Pressing the print screen button will allow you to select the captured area.
Pro Classic UI design makes it easy to use.
Pro Selecting custom rectangle is the default
Easy to use selection tool with a magnifier to select only the part of the screen that you need.
Con This version only works on Windows, not Linux nor Mac
However, the same author now has a separate version of Greenshot for iOS, which is available (for a small fee) in the Mac App Store at https://itunes.apple.com/us/app/greenshot/id1103915944