Low-flow shower heads (sometimes called energy-efficient shower heads as they also use less energy, due to less water being heated) Buy here.
Low-flush toilets, composting toilets and waterless urinals, which can have a dramatic impact in the developed world, as conventional Western toilets use large volumes of water.
Faucet aerators, which break water flow into fine droplets to maintain "wetting effectiveness" while using less water. As a bonus, they reduce splashing while washing hands and dishes. Buy here.