When I add and existing class to a project I just noticed the "Add as Link" option.
I have classes that I use in different projects (SelectionTools.cs, BlockTools.cs etc.).
If I add them as links then any features I add (or more likely bugs I fix) are there for all projects with that class.
Better than having various versions of the file floating around.
I read where C++ projects do this by default. Seems like a good idea.