Desdichado
Hero
I feel a bit uncomfortable recommending something without having much of any idea of what you like or what you're looking for, but here's what I would recommend as most useful to me in developing settings I like:
- Midnight campaign setting -- great alt.rules and great design to use as inspiration.
- d20 Call of Cthulhu -- tons of stuff to steal from here. Even the Sanity mechanic would be useful in a non-CoC campaign, I think.
- Monsternomicon -- great new monsters, lots of flavor. This almost does it for me in terms of monsters. I do need more fiends, though. Which brings me to...
- Green Ronin's book of Fiends. I have the first two (the one's that are released already) and count them amongst my favorite d20 books. Combining them with a third book and updating them to 3.5 is just gravy.