V-Control Pro works for me, though I mostly use it to control Sonar's console rather than plugins,
Another possibility is Microsoft's (free) Windows Remote Desktop application, though that drops the PC screen back to the login screen when the main display is redirected to the remote display. Still quite useful in all kinds of ways though.
Personally I'd like Cakewalk to come up with a remote tablet application of their own, along the lines of Apple's Logic remote controller.