Custom XML Comments
Both C# and VB.NET support XML comment notation (/// or ''') in 2005 and, when a project is compiled in Visual Studio 2005, a file is created that contains a complete listing of all comments made for classes, methods, properties, etc. This is a great feature because it immediately makes available all of the documentation into one file.
One of the things I played around with was the ability to add additional XML tags. For example, in my Data Access Layer, I added a <storedProcedure> element, which compiled to the XML documentation file as well. This way, I also knew which stored procedures I was actually using in my data components. That is a great feature.