In software projects, we have files we actually edit and maintain, and files that are generated - by compilers, pre-compilers, build systems etc. When using an IDE, you very often just want to see the files you might want to edit, not the generated ones (even though they might be perfectly editable themselves).
Now, I've been using Eclipse's Resource Filters mechanism to hide generated/temporary files in my Project Explorer. But actually, already have a clear marker of which files should be visible and which should: The files tracked by my source control mechanism - Mercurial, Git, SVN - etc. And Eclipse is actually aware of what's tracked, through the appropriate plugin (SubClipse, MercurialEclipse etc.)
So, is it possible to get Eclipse to only show tracked files (in the Project Explorer)?