The Plugin Spy gives you easily information about the running UI. Press Alt+Shift+F1 to get information about the current running Eclipse plugin / data types / screen. This way you can get immediately access to the plugin which is currently running.

Click on the active class or the contributing plugin to get to the class or the source code of the class.

Press "Alt + Shift + F2" and select a menu to see who is contributing this menu.

SWT spy for Eclipse is a tool that prints out information about the widget under the cursor. Currently, this includes style, layout and parent information. See SWT Tools for more information.