[Tristan Lall]

NQC is good (or at least it was, about 8 years ago, when I last used it).

There's also an ActiveX control you can use to issue commands to an RCX over the infrared port (from a computer's serial port). It can issue any command to the RCX (including telling it to a program stored on the RCX), so you can use any familiar programming language.