30 Days of Worldbuilding - it's a pretty good resource in general and doesn't have to be done over 30 days.
http://www.mortaine.com/personal/MagicalWorldBuilder.pdf
This hideous website has a wonderful archive of Patricia C. Wrede's thoughts and ideas for worldbuilding. It's great for sparking off whole plotthreads.
http://www.larseighner.com/world_builder/
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