The Perfect Flush turns any standard toilet into a push-button dual-flush toilet. It installs in just 30 minutes and, according to the manufacturer, can reduce a toilet's overall water use by up to 50%...
The Perfect Flush by Brondell works by automatically controlling how much of your reservoir is used with each flush (half or full). It is designed to fit a wide range of toilets, is powered by 4 AAA batteries (good for 20,000 flushes), and currently sells for $99 direct from Brondell.
Their claim is that the money saved on your water bill will pay for the cost of the device in about one year.
One commenter on Good Clean Tech mentioned that he has installed a solid connection between his handle and the flapper valve, allowing him to manually control the amount of water per flush and do a half flush, full flush or even quarter flush. Interesting idea.