Thunderbird Add-ons at addons.mozilla.org is great way to start browsing Thunderbird extensions.
Writing An Extension
See the Mozilla Developer Center for help with writing extensions. There is a list of links to helpful development tools. The Thunderbird page has a tutorial, some examples, and links to other sites with resources for Thunderbird extension developers.
Better extension APIs
New APIs are planned at the Thunderbird:Extensions:Needed_APIs page.