By popular demand, I tossed Plug-in Spy on a temporary update site for people to try out and give feedback. Note, it needs Eclipse 3.3+ and to try it out, simply select things and press F9. If you have ideas, feel free to file bugs or enhancements against the PDE Incubator with the tag [spy]. I recently added the support of getting at those pesky menu ids you need to contribute to Eclipse menus (thanks Paul Webster).
Note, this is incubator code and there is no guarantee of stability or “product readiness.” Maybe given another bloody mary or two, I can add more functionality ;p