Toilets that offer two different flush volumes are the norm in much of Europe, but they've been slow to catch on stateside because the higher initial cost tends to offset any savings on the water bill. One2Flush offers a cheap water-saving alternative to installing a new toilet: a kit to upgrade your existing toilet into a dual-flush model.
The kit provides all the parts needed to add dual flush capability to most standard toilets. Once installed, pushing the handle down results in a half-volume flush for liquid waste while pushing up yields a full-volume flush for, well, you get the idea.
Installation is fairly straightforward and requires only basic hand tools, but it does require taking the tank off the toilet bowl which can be intimidating to those without much plumbing experience.
The kit retails for around $25 at various big-box chain stores and on-line retailers.