An introduction to writing modules by Nick Fortune (aka boroshan) can be found here.
Some useful Fvwm modules that are not included in the main Fvwm distribution.
- Felidae
http://www.florian-diesch.de/software/felidae/ - FvwmExpose (dead link, moved?)
http://dev.gentoo.org/~taviso/files/FvwmExpose - FvwmKb
http://freshmeat.net/projects/fvwmkb/ - FvwmMonitor
http://www.fvwmforums.org/phpBB3/viewtopic.php?t=325 - FvwmPartition
This module provides Ion like window placement for Fvwm
http://www.imonk.com/baboon/FvwmPartition/ - FvwmPy
http://barry.warsaw.us/software/fvwm.html - FvwmScriptMux
Link is dead but I still have a version available here.
http://alex.foxybanana.com/fvwmscriptmux.html - FvwmTabs
This is part of Fvwm now.