GNU
GNU's not Unix!
GNU is a recursive acronym for "GNU's Not Unix". GNU was started by Richard Stallman in 1983 to make a completely Free Unix replacement. It has succeeded with GNU Linux and will probably succeeed with GNU Hurd. GNU provides many of the UserSpace tools for GNU Linux and other Operating Systems.
GNU apps nowadays range from compilers and kernels to end user tools for editing and music. To keep up with what the GNU community is doing, you may find this large unofficial GNU Forum useful. This forum (hosted by Nabble) currently archives all the gnu projects' mailing lists into a hierarchical forum for easy browsing and unified seach. You can drill into each project or search a keyword and find out what the GNU folks are talking about.