Developing Movable Type Plugins
Timothy Appnel has published a fantastic guide to developing plugins for Movable Type. He discusses adding simple tags to the templating system, creating text formatting plugins, and using the data persistence framework to store plugin data. This is a great tutorial for beginning plugin authors, and will hopefully open up plugin authoring to a new group of developers.
In other plugin-related news--if you haven't seen it recently, mt-plugins.org is the best resource for finding and using MT plugins. A bugtracking system was added recently to get users in touch with plugin developers about problems with the plugins.