Richard Stallman (or in geek circles simply rms) is an outspoken advocate of Free Software. One could argue that without his pioneering work founding the GNU project that things like Linux and Firefox wouldn't exist today.
He is also a controversial figure who can and does divide opinion in the wide diaspora that constitutes the world of FLOSS. Not given to compromise on his core values of freedom he is often at loggerheads with the more pragmatic proponents of Open Source or the "more free" BSD style licenses. Although he's often criticised for perceived obstinacy many admire him for sticking to his unwavering principles even if it causes him inconvenience.
Today I noticed that he is coming to Manchester at the start of May. I've never seen him give a talk before so I'll be there on the day. If you only see one bearded hacker talk in your life then this would be the one to see.