If you press Shift+Ctrl+T or choose "Navigate > Open Type..." you get the "Open Type" dialog for quickly navigating to a known class. When you start typing a name only the classes for which the name matches stay visible. That way you can find a class of which you know the name very quick without having to browse through the package explorer tree.
This has been working great for me up until this morning. All of a sudden for a couple of my projects I am only seeing some of the types that exist. Of course I tried the obvious steps of refreshing the projects, cleaning the projects, re-building the projects, rebuilding the projects externally, but all to no avail. It is a bit odd since the types are known in other places. If I add an import statement Eclipse does not complain that it doesn't know the type and I can Ctrl-Click through the types to get to their file. However, the type navigation knows nothing about them.
In the past when InteliJ used to do this to me I would go find its cache files and delete them forcing it to rebuild. Does Eclipse have something similar I might do (I'm an Eclipse newbie)? I am using Eclipse 3.4.2 and I have it configured to not delete files on a clean (because our actual build process puts files into the output directories that I don't want Eclipse mucking with).