We were recently troubleshooting an installation of a Windows Forms Application.  We couldn't run the executable.  We figured we were missing a dependent assembly, but didn't have a fast way to figure out which one.  We came upon this great tool that quickly pointed us to the missing assembly.  Not only did it help us to figure out which assembly we were missing, it helped us to determine which assemblies we were including that we didn't need to.

CheckAsm is very similar to Dependency Walker (depends.exe), but for .NET assemblies rather than system assemblies.  It saved us a lot of time and is really easy to use.  It is a great free tool.

http://checkasm.booring.net/