The DevKit Solar Controller is a powerful, low cost solution to monitor and control a wide variety of things. It has also been successfully tested with a solar hot water system. This little device has Built-in-Functions for fast deployment, is easily programmable in BASIC, has an LCD display (3 buttons, 3 LEDs), comes with an External Antenna with a range of over 100m allowing for sensory data retrieval in areas where wired communication may either be unfeasible or impractical, and has a customizable sensor board. It is powered by an internal rechargeable battery, 5V USB, as well as from Solar Power. The solar power enables the battery to remained charged in case all power is lost or at night.
Hot water solar controller and data logger. Switches pump and measures temperature differences and flow rate.
Industrial Fan controller in combination with the AIRcable Serial5. Sends on/off, faster slower commands to the fan wirelessly, displays temperature and speed.
More examples are on OpenSensors.org.