The RCX brick is build upon the Hitachi H8300 chip which is supported by both gnu binutils and the gnu compiler, gcc. This is particularly cool, because some german folks used this to program legOS, a microkernel / operating system for the RCX brick. With this setup, you can use gcc to compile C and C++ programs to control whatever robot you can make out of the Lego bricks. If you ever wanted to code a neural net for your Mindstorm robot, this is your chance.