Hollow's Last Hope
d20srd.org Pretty much the defacto site for System Reference Document info on D&D.
D & D Wiki same content as the link above
Paizo Publishing Home of the company that produces a ton of kick ass stuff
Invisible Castle If you feel the need to roll online or just don’t have some dice with you
Pen, Paper, Pixel They have a nice virtual dice bag and other stuff like character sheets, etc
RPG Xplorer Decent software for managing characters and so much more. Free up to level 5.