How to run regasm.exe from command line other than Visual Studio command prompt?

In command prompt:

SET PATH = "%PATH%;%SystemRoot%\Microsoft.NET\Framework\v2.0.50727"

You don't need the directory on your path. You could put it on your path, but you don't NEED to do that.
If you are calling regasm rarely, or calling it from a batch file, you may find it is simpler to just invoke regasm via the fully-qualified pathname on the exe, eg:

c:\Windows\Microsoft.NET\Framework\v2.0.50727\regasm.exe   MyAssembly.dll

Like Cheeso said:

You don't need the directory on your path. You could put it on your path, but you don't NEED to do that. If you are calling regasm rarely, or calling it from a batch file, you may find it is simpler to just invoke regasm via the fully-qualified pathname on the exe, eg:

%SystemRoot%\Microsoft.NET\Framework\v2.0.50727\regasm.exe MyAssembly.dll