You know the kind - candles that have been used so much that the wick is down to the quick even though there's still wax left at the bottom. The glass is more than likely blackened by the hours it spent lit. It should probably just be tossed. But you love the sent!
Don't cry, and don't toss the candle yet.
Melt the wax in the microwave on 50% power until it is liquid. Then carefully, with potholders, pour the wax into cupcake holders.
That's it! After the wax hardens again you can peel away the paper and pop it into your candle wax warmer. This way you still get to enjoy your favorite sent long after the candle life has passed.