I've just finished and published my very first add-on for Mozilla Thunderbird: Filter SubFolders, a plugin that applies your mail filters to sub folders of an IMAP account, not just the inbox. It does so automatically when new mail is detected, and manually through a menu item.
Indeed, this is the first project I've ever OpenSourced. That sounds weird for a FOSS geek, but what I mean is it's the first thing I've created from scratch (well, mostly) and published for the world to see as OpenSource. Sure, I"ve contributed to other FOSS projects in the past, but this is my first "novel" contribution. And it feels good.
The extension is going through the review process at http://addons.mozilla.org/ now, but if you want to get it early, it's available at https://addons.mozilla.org/en-US/thunderbird/addon/filter-subfolders/. Usage details and help are at http://www.stroppykitten.com/filterSubFolders/.
Ultimately, I did it for myself (I had that itch to scratch), but hopefully it's useful for more than just me.
Have at it
UPDATE:
I don't use Thunderbird anymore, and I have no more need for this add-on, so it's now un-maintained. But feel free to grab the code, use it, update it, or take over maintenance if you see fit. It's OpenSource y'all :)