Did you ever try to implement plugin in Eclipse? If you didn't you can probably skip this post.
But if you did you know that Eclipse PDE is very nice. It provides you a lot of neat wizards that help you to start. However, you often experience a situation, when you want to write a feature that is "almost like that feature that already exists, but...". So what you're lacking is the code of "that feature", so you be able to look on code and do the same. But how the hell can you find this code?
Yes eclipse is open source and yes it comes with all the sources. But it has dozens and dozens of plugins and it can be exhausting to find the relevant plugin.
And here comes the "Plugin Spy", which was added in Eclipse 3.4. Just click Alt+Shift+F1 on the proper place in the workbench and it will show you a lot of useful information. It may be still not easy to find "that feature", but at least it gives you the directions where to start.