As someone who's hacked up a custom editing interface through older editions of MovableType, Kevin Shay's BigPAPI plug-in is a welcome edition. Writing the backend of a user facing extension is relatively painless, but integrating into the standard MT interface was a relative pain. BigPAPI, as described in Anil Dash's overview, is "server-side Greasemonkey for Movable Type". SixApart's pro developer's network, also has a good BigPAPI tutorial, penned by Shay.