RMS is short for 
Richard M. Stallman.
Look at my HackersBeard, everyone! -- RMS
Richard Stallman wrote Emacs, gcc(1), make(1); all sorts of the useful stuff on a Linux system. He did this as part of the GNU (GNU's Not Unix) project, to try and replace the commercial Unix with a Free OperatingSystem.
Opinions about Stallman as a person tend to be sharply divided. On the one hand, he has been nothing less than a visionary in promoting the idea of FreeSoftware, and he was one of the first to sound the alarm over SoftwarePatents. But on the other hand, he seems to have a propensity for what can only be described as control-freakery, which was a factor in the emacs/xemacs schism, the EGCS fork from GCC (since reconciled), and accusations of trying to discourage Linux development in its early days in favour of his own Hurd kernel.
