Eclipse PluginSpy has been there for sometime now. Select any view or editor and do ALT+SHIFT+F1. This will open the plugin-spy popup window and shows information about the view/editor selected. It shows information about Active Shell, Active Part, Active Selection and Active Help. Try it if you did not know about this feature.
What's new in Eclipse 3.5 is, PluginSpy now works for toolbar and menu items also. Do ALT+SHIFT+F2 and move the mouse over toolbar or menu item. You will see the cursor change to a book like image. Now, click the toolbar or menu item and you will see that plugin-spy popup window come up for that also. This shows Active Selection and Contributing Plugin Information for the selected toolbar or menu item.
I find it very useful. I'm sure every Eclipse Plugin Developer will appreciate this great feature.