More to be added soon. In the mean time:
Kineme.net - this site is mainly for QC plugins, they have a lot of VERY useful plugins, tools, and info. If you're doing anything with QC, you should absolutely know and regularly visit this site.
There's also a very good forum, probably the most active QC related forum, and a good place to ask questions.
Quartzcompositions.com - this site has 3 main areas, a repository for compositions (not great, but worth a look through), a forum (unfortunately almost dead), and a wiki with a lot of excellent info on QC. The wiki is highly recommended, you can learn a lot there!
Apple's Quartz Composer Mailing List - there's a ton of useful info buried in the mailing list archives, they're a great resource you should search if you're not sure how to do something. Also sign up to the list if you want to take part and ask questions - some of the QC developers at apple are on the list, so it's a great source of good info.
Apple's Quartz Composer Programming Guide - essential reading if you want to write an application using QC (which is essential if you're making a real demo).
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