There are plenty of worthy water-saving devices. Installing a restricted flow shower head is one of the most reliable ways to take a bite out of your monthly utility bill. So is upgrading to EnergyStar appliances.
But you can save thousands of gallons of water without spending a penny on upgrades simply by being smarter about your use. Don't run a water-using appliance until it's full (e.g. dishwasher, washing machine). Learn to take shorter showers. Flush only when it's necessary. Water your plants when they need it — not on a schedule. Common sense is the ultimate conservation tool. The more you use it, the more you save.