There's also a ring gag (which I found uncomfortable for long periods) but my favourite is just a piece of cloth similar to a face washer, scrumpled into a ball and shoved into the mouth, then maybe having some saran wrap or a bandage or something wrapped around to keep it shut.

I suffer from TMJ or lock-jaw, so find I can't use ballgags etc for long periods as my jaw gets very painful and locks itself into place, but with a cloth gag inside the mouth, I find I can still move my jaw around a little bit and can be kept like that for a while. Certainly can't hear anything intelligible.