Ray Wang describes standalone water scheduling controlled via web browser or mobile app. It is also expandable by adding additional units.
Available as a fully assembled product, or a DIY kit (including PCB, components, and enclosure to make your own OpenSprinkler).
additional information after the jump..
OpenSprinkler is an open-source Internet / web-based sprinkler timer / controller built upon the Arduino platform. It works with standard 24VAC sprinkler valves commonly found in household watering and irrigation systems. You may be planning to buy or have already purchased a commercial sprinkler timer / controller, but chances are that these systems provide a limited set of fixed functionality, and have no web scheduling and monitoring capability. These limitations are the main motivations for this open-source project.
via OpenSprinkler — Open-Source Internet-based Sprinkler / Irrigation Timer « RAYSHOBBY.NET.