UPDATE: Flag Framing and The Conflict Web— two tools to make improvising in any game system easier (and more powerful).
Dragon Generator Makes dragons in two picks and one click! (Works in IE only).
Links for GMs Lots of general GM resources.
EN World’s Online Gaming Toolbox, requires login to see them, but they’re pretty extensive. Unfortunately, some links are broken.
A guide to creating detailed cultures and fantasy races.
Handy PDFs for D&D; mostly variants and interesting house rules.