These programs aren't strictly Mindstorms related, but they may be of interest to Linux/Lego fans.
LeoCAD is (as the name implies) a CAD program for constructing and renderging Lego models. There is a usable GTK port, which can be found at http://leocad.gerf.org/linux.htm. The author implies that it doesn't have full functionality, so this might make a fun project for someone.