Although a lot of software is available for GNU/Linux and not for Windows, I am always happy to see new ones developed for GNU/Linux but not for Windows (at least not yet). One last example: Zfone. This software “intercepts and filters all the VoIP packets as they go in and out of the machine, and secures the call on the fly”. The public beta version is already available for GNU/Linux and MacOS. Windows version will be available in mid-April (only). It would be great if Philip Zimmermann releases the source code under a free software licence (other comments and ideas in this Slashdot news).
P.S.: At FOSDEM 2005, Richard Stallman used the words “GNU Linux” to insist on the fact that software from both “groups” were associated. At FOSDEM 2006 (videos here), Richard Stallman went a step further and used the words “GNU/Linux” (pronounced “GNU slash Linux”). See also this Wikipedia article on the GNU/Linux naming controversy.