I'm currently involved in
two world building projects. The first,
Urbis, is strictly my own show. The other,
Pisces, is a collaborative effort (basically, anyone who wants to contribute something can do so at this stage) - though I did come up with the name and basic idea for the setting.
For
Urbis:
The Core books, Psionics Handbook, Epic Level Handbook, Manual of the Planes, as well as various game products from different RPG systems (most notably GURPS), and lots of non-gaming books (most notably the Dictionary of Imaginary Places). The
Obscure Cities comics were also a major source of inspiration...
For
Pisces:
Again, the Core books and the Psionics Handbook. Personally, Blue Planet has been a major inspiration (for the bizarre but plausible alien life forms).
The others probably have lots of sourcebooks they get inspiration from as well... but you'd have to
ask them yourself...