How can I manually change default applications in OS X? I'm facing an issue with Transmission's "Set Default Application" button - it seems to do nothing. Magnet links still don't work, so this feature is broken for me. I have been able to use Magnet links with uTorrent prior to switching to Transmission, so I know there's no issue there.

Is there an app or any manual way to set/change protocol associations without having to use the target application to set it?

I'm using Transmission 2.42 (13013) running on OS X Lion 10.7.2.
RCDefaultApp is a Preferences Panel which allows you to edit the default application used for various URL schemes, file extensions, file types and MIME types.