I went to a sewing party and one of the ladies made healthy snacks. She air-popped some popcorn and brought different toppers for us to shake on:
taco seasoning
old bay seasoning
parmsean (note: real parmesean is also low and fat and because it is high on flavor, you use less so it's worth the splurge, I think)
Cajun seasoning
JELLO powder! *this was a great idea and mimics those gourmet candied popcorns
Dried broccoli-cheese soup mix
NORI (Dried seaweed)
Onion Salt
Dill Salt
Salt/Sesame seeds
Now, I haven't tried this but I bet you could even make a low-fat caramel sauce with a low-fat "butter" and brown sugar (brown splenda) Or better, yet, drizzle a container of that low-fat caramel apple dipping sauce on your popcorn.
Stay away from microwave popcorn. It gets its buttery flavor from diacetyl, a dangerous chemical that may cause "popcorn lung"
http://www.msnbc.msn.com/id/20620395/