Best Open Source Software

This is a BOSS thread, let's see what turns out to be our favorite open source software. For me, it's definitely GIMP, the gnu image manipulation software. You can get it at http://www.gimp.org/

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Name one thing that Photoshop can do and the GIMP can't do.
You can't.
You're just too lazy to learn the interface and to configure a window manager that follows the standards.