I like experimenting, so I usually pop mine from kernels so it is actually plain. In the past I've done and enjoyed:
garlic parmesean
garlic dill
mrs.dash original
cinnamon and equal/splenda
seasoning salt
garlic pepper
sea salt and pepper
lemon pepper garlic
carribean jerk
mediterranean blend with sea salt [by mccormack]
pretty much anything in my spice cabinet is fair game. I've even done a spray of olive oil with a little lemon juice, parmesean and garlic.
Then if I do ready-pop, I do the 94% Smart Pop Kettle Corn
Hope those give a few ideas