Markus L. Noga
Developed on x86 GNU/Linux and tested on PPC Linux. It has also been ported to Cygwin and DJGPP on MS Windows. Solaris and Irix ports have been attempted but not all tools work.
LegOS is a pre-emptive multitasking POSIXish OS for the RIS. Programs are written in standard C or C++, compiled on the PC using gcc (built as a cross-compiler), and then downloaded to the RCX where they are executed. Basically, anything you can write in C or C++ (and 32K of RAM, of course ;) you can write in legOS. Interesting features include random(), floating point emulation, threading with POSIX semaphores, and the ability to store multiple programs. It also includes functionality for sending and receiving data from Linux and MS Windows PCs. This power (legOS is almost definitely the most powerful of the RCX alternative software systems) comes at a slight cost: because it uses gcc, legOS is probably the most complex system to set up of the various Linux alternatives, and requires the largest download of tools.