Here are some modes/add-ons for Emacs that I found useful enough to write code for them (or to adopt and maintain them as is the case with prolog-mode). Please note that I use (and develop under) Linux and Mac OS, so some of the code below is not tested on anything else than Unix-like systems.
All the packages are made available under the GNU General Public License as published by the Free Software Foundation.
emacs-pilot syncs Emacs stuff (BBDB, diary, memo transfer) with a Palm
OS device.
emacs-prolog is a major mode for Prolog programming under Emacs and offers to my knowledge the best Prolog IDE out there, especially when used from within XEmacs.
music.el (current version: 1.16) is a major mode for managing a multimedia library. Works with most CLI and
GUI based multimedia players, though the interface is slightly different in for the two classes – see comments in the source file). I started to work on this one when I could not find any decent management tool for my music library (I have converted most of my CD collection to MP3 tracks). I have meantime found some (XMMS most notably), but by then my hack evolved enough as to become more flexible than them.
If you improve/change these files, I would appreciate if you send me a copy. Not only I will be glad to use it but, unless you tell me not to, I will post your changes here, for others to use (and praise you).