If you are like me you still use the Search dialog for plugins to find the definition plugin of an extension point.

I just realized that the Open Plugin Artifact Dialog (Ctrl+Shift+A) allows much faster access.

I also added the shortcut and a description to my Eclipse Source Code – Tutorial.
About Lars Vogel
Lars Vogel is the founder and CEO of the vogella GmbH and works as Eclipse and Android consultant, trainer and book author. He is a regular speaker at international conferences,
With more than one million visitors per month his website vogella.com is one of the central sources for Eclipse and Android programming information.
Just a minor addition: If you currently are in the plug-in manifest editor and already have extensions contributed, you can also use the context menu to “Find Declaration”.
But I assume you are aware of that.
@wwerner thanks for the tip.