I think that there has been at least one thread on this before, but that's not to say its not worth re-visiting. I'll start the ball rolling with boring ( ) old clothes pegs, elastic bands and string.

A very efficient gag can be made by securing two pencils together with elastic bands at either end and pushing the tongue through them. This keeps the tongue pushed out, restricts speech (and screams), hurts and, best of all, allows the submissive to drool everywhere.