Similar to jumpingbeans's idea...rub a potato on it, and bury the potato in the garden
Tea Tree oil has worked for several people I know. I works even better when mixed with equal quantity of lemon oil. Use neat essential oil. Dab on a drop 4 times a day.
Keep using it after the wart has disappeared, as they will probably still be some remnants deep under the skin that you can't see. Both oils are very cheap (a couple of £ for a decent brand) and a bottle lasts for ages and ages.
Its seems to help if you "file" the wart down every couple of days too.