I spend a lot of time with the XmlSerializer (I personally dig it immensely, and I think too many people complain about it, but anyway) and while I put up an article on how to debug directly into the generated assemblies, I noticed that Mathew Nolton has a GUI Front-End to Chris's XmlSerializerPreCompiler.
The tool will check to see if a type can be serialized by the XmlSerializer and shows any compiler errors that happen behind the scenes. +1 for Useful, thanks Chris and Mathew.