I’m currently writing Eclipse plug-ins, leveraging the JWT project.
There’s a great book for learning how to program plug-ins, and they have a site for it: http://www.qualityeclipse.com/
If you use Eclipse, you should read Chapter One on the site (free); even if you aren’t programming plug-ins! They go over all the features of Eclipse that you should know about. There’s plenty of interesting reading in the other chapters – the previous IDE technologies that made their way into the app, how plug-ins work, how to build a basic plug-in, using SWT to develop great GUIs.
I’ll be releasing a project soon, complete with a help section, an update site, and all that you’d expect from an Eclipse plugin. More on that in June!