TONS of free patches of every variety, going back years, because it's...
Used by everyone from UI designers to screensaver creators to sound professionals. They'll connect up QC with Pro Tools and other sound equipment to coordinate stuff on screen.
Origami 1.3 solves a lot of common usability problems
Avocado by IDEO also adds some pretty impressive stuff
Thanks a lot for linking to my QC video course (http://scotthurff.com/qc)!
Here are some other pluses for QC:
Also note that Xcode isn't required; QC is free with the Graphics Tools: https://developer.apple.com/downloads/index.action