OWFS -- 1-Wire file system. Use the Dallas 1-Wire and iButton chips with standard filesystem commands. Create temperature loggers. Monitor everything. OWHTTPD -- same system, only used as a light weight web server. OWFS is also ported to embedded routers, Mac OSX and Windows.
Basically you can use these inexpensive little sensors and other chips with very simple wiring to sense and control the physical environment. The interface is very flexible -- everything from a web browser, to read and writing directly to the chip's "file" to integrating with your programs -- even over the internet.
There is a growing number of 1-wire sensors: temperature, voltage, humidity, light, ultraviolet, pressure, thermisters, thermocouples, programmable microcontrollers, relays, lights, LCD panels, etc.