Resources on creating plugins

Overview:
http://textbook.textpattern.net/wiki/index.php?title=Plugin_Development_Topics

Tutorial: 
http://thresholdstate.com/articles/3975/anatomy-of-a-textpattern-plugin-part-1

Mailinglist for plugin devs:
http://lists.textpattern.com/

Developer resources (further links, tools and help):
http://forum.textpattern.com/viewtopic.php?id=9881