Did you know you aren't supposed to burn a candle down past half an inch? (Flip over an unlit candle and look at the warning sticker on the bottom.) Apparently the glass container can shatter.
Anyway, I wanted to re-use a candle container and found a wiki that suggested putting the candle in the freezer for a couple of hours. I did and much to my surprise, the wax popped right out when I turned it over. Super easy!