Diaspora Homepage - Our home base.
Hard Light Productions - The location of our forums. You may find the FRED Forum particularly useful for asking questions as that way you can get questions answered by the FREDders for several FS2_Open games and mods rather than just Diaspora-based FREDders.
Freespace Wiki - Despite the name it has a very large amount of information on how to mod and FRED for any FS2_Open game including ours.
Karajorma's Freespace FAQ - The FRED section has a load of hints and tips on how to get the most out of FRED.