(Disclosure: I work for this company.)

The Teleo system by MakingThings (www.makingthings.com) is designed to be an easy to use, inexpensive, modular and extensible way to easily interface various sensors (temperature, pressure, light, switches, etc.) and actuators (motors, lamps, relays, valves, etc.) to a computer. We provide a simple C API that runs on Mac OS X, Linux, and Windows.

Please feel free to contact me if you have any further questions.

Sincerely,
Michael Shiloh
michael@makingthings.com