We keep microwaveable popcorn (usually Pop Secret brand) popcorn around for an occasional late-night snack while we're watching a movie. I would say we have it's once a week at most.

A few years ago, I saw one of DH's relatives melting a little butter to put on some popcorn, and it was a an eye-opener for me---it seemed awfully decadent. (I grew up in a household much more given to self-denial than self-indulgence.) Now I always melt a little butter for popcorn. As someone said above, it's not much in terms of calories, and just a tablespoon or less is plenty for enough popcorn for two people.