Eric S. Raymond. (Commonly referred to as ESR). Hacker.
Author of the book 
The Cathedral and The Bazaar; the book that introduced a lot of the corporate world to the concept of OpenSource.
See his homepage at http://www.catb.org/esr/.
Maintainer of the JargonFile.
Although RMS won't say it, there are many in the FreeSoftwareFoundation willing to call him pure, pure evil. Some suggest that he undercuts the moral principles of FreeSoftware by running the OpenSourceInitiative.
He wrote fetchmail(1), and a program to convert gifs to pngs.
He's not half as hairy as RMS.
