From now until June 30th, Art of Beauty, the makers of Zoya nail polish (arguably the best nail polish on the planet, vying neck and neck with OPI these days) will take your old, unwanted nail polish colours and exchange them for your choice of any of the Zoya colours they have (of which there are a daunting number of choices).
The cost is $3.50 per bottle for shipping/handling/exchanging. As many bottles as you want as often as you want.
This is a fantastic deal for those old gift polishes that you never wear or the nail polish you THOUGHT would be an awesome colour and turned out to be very much not. Zoya polishes are usually about $7 each, so you're saving $4 per polish plus shipping.