I have to admit, I only use a small portion of the features that are in Eclipse. That's because there are so many features that I often miss things, even buttons that stare me in the face everyday. I love Eclipse, it's a really powerful IDE and one of the most impressive uses of client-side Java that I've ever seen.
Yesterday Michael Henke posted a nice entry (complete with screenshots) on some of the features of the Eclipse Navigation Toolbar. I've used the "Go Into" feature quite a bit. If you have a large project that you're going to be working in exclusively, it's a nice way to keep your navigation tree to just the relevant files and folders.
However, I must admit that I've never taken the time to figure out what the "Link With Editor" button does. I've clicked it once or twice, but never saw an immediate purpose for the button. Thanks to Michael's post, I know now that the clicking the "Link With Editor" toggle button will cause the Navigation pane to automatically jump to the active file open in the current editor tab.
Since I often have many tabs open at once (sometimes even dozens) I can see this feature being useful. Especially since often related files are grouped together in my project.