Not Quite C is Dave Baum's (and several others) contribution to Mindstorm programming. NQC is available from Sourceforge and provides a command line programming environment. Bricx is a GUI IDE provides a platform for working with NQC and some other languages for programming the RCX.
The LegOS alternative to the Lego RCX opererating system has been renamed the brickOS and the homepage has been moved.
The LeJOS is another alternative to the Lego RCX operating system that is written in Java and supporting a Java VM. The LeJOS homepage at sourceforge is well-organized and speaks for itself.