MLCAD
MLCAD is a great 3D CAD program to create LEGO models. It is available
here.
POV
POV is a raytracing software to generate nearly photorealistic pictures of 3D models. It is available
here.
NQC
Not Quite C is a programming language to program the Lego RCX. You can find it
here.
RCXCC
RCX Command Center is an IDE for NQC. The name has changed, it is called Bricx Command Center now. You can find it
here.