My fellow collegue here at geekswithblogs.net states a way to add assemblies to Visual Studio's Add References dialog box.
I would recommend NOT using the first method described, copying into the IDE/Common Assemblies folder. This is reserved for Visual Studio's OWN assemblies that support the IDE and should NOT have random assemblies copied into it. For starters, determining the correct version of the IDE is difficult and may change.
However, use of the second method, registering in the .NETFramework\AssemblyFolders registry key is the RECOMMENDED way. This provides a generic, Microsoft .NET Framework endorsed way of telling the current system that your assembly is available.
For those who want to know, its the registry key:
HKLM\Software\Microsoft\.NETFramework\AssemblyFolders
PS: Hope everyone had a good Thanksgiving!
UPDATE! Ironically I was just reading blogs.msdn.com, and first up was this post regarding this very topic: Adding assemblies to the Add References dialog in Visual Studio 2005 He talks about a more framework version specific way and a current user specific way.