soy sauce is actually very good on popcorn.
make some in an airpopper and dump it all in a paper grocery bag. dump in some soy sauce and shake it all up to coat it then dump it back into a bowl.
other fun combinations: garlic powder + chili powder came out good.
we tried an "italian" mix once that was gross...oregano, basil, and a little garlic powder. good in sauce...not on popcorn