Note: Please do not remove useful links, unless they are harmful or illegal.
The Hypertext d20 SRD Great version of the SRD.
Random Dungeon Generator Worth a look, really interesting stuff.
Official Planescape 3.5, free, legal pdfs Fun pdfs. Planescape connects to all planes, including the one the Black Marches are in
Official Spelljammer site Spelljammer connects all Prime Material planes, including the one the Black Marches are in
Official Dark Sun 3.5, free, legal pdfs Another fun pdf. A desert setting, an alternate Prime Material plane. Somewhere to travel to in a planar/spelljamming adventure.