NTFS junction point
An obscure, not very well known OS feature since Win 2000, the NTFS Junction is basically like a virtual or mapped folder so that you can have one folder in multiple locations at the same time, but in reality are just one folder, with one file collection. This is done at the kernel level, so it's invisible to all applications, including IIS, and Visual studio.
Why am I bringing this up, well, recently I was trying to find a way to share master pages across applications. First thought, was copmile the file into an assembly, chuck it in the GAC... but this leads to having files all over the place that inherit this class...but don't do anything else. Dummy files. Waste of space and everything else. You need a physical master file to allow desiner support and runtitme support for a master file.
So what do we do? Well, with a little help from Scott Guthrie and David Ebbo I was introduced to the Junction Point. So, if you create junctions in your projects, you can trick your apps and visual into thinking the master is there... well, for all intents it really is there, but many apps now have use of these files as if they where local files.
You can use this technique to share user controls as well, or images, or whatever you want really.
This is WinBolic, I found it to be a very easy and nice tool for creating the junction points with a UI interface, of course you can make them in command line as well.