New and updated! Now finds obsolete files from Visual Studio 2008 projects.
Recently I came across several dozen projects that had been worked on by dozens of people for several years. Inevitably, there were files present in the source tree that were not part of the project.
** Update: The tool can now be downloaded here. It used to be on CodePlex, but it's not any more. **
To use it is very simple. Download it and run it, or right click a Visual Studio project in Explorer and 'Find Orphaned Items...'.
I've tested this project on Visual Studio 2003 and 2005 projects. I haven't tested this on Web Projects or Web Application Projects (in Visual Studio 2005). I would recommend verifying what the application thinks are orhpaned items. If it continually gets it right, you can trust it more and check it less. If it continually gets it wrong, then please let me know!