Thermocouples find widespread use for temperature measurement in systems. During system design or testing, you must observe the system's response at different temperatures.
http://www.edn.com/contents/images/110801di.pdf
A thermometer with a computer built-in. It can be programmed in a basic-ish language. Scripts for making a graphing thermometer and a GSM cell-phone connected vending machine are provided.
http://www.riccibitti.com/scriptherm/scriptherm.htm
http://www.lancos.com/webtherm.html
a heat activated sensor
http://www.mitedu.freeserve.co.uk/Circuits/Misc/tempalarm.htm
http://www.cs.unibo.it/~lanconel/therm/schematic.gif
platinum resistance-temperature-detector (RTD) sensor provides an accurate means of obtaining temperature readings
http://www.edn.com/archives/1997/060597/12di_04.htm
A simple yet accurate thermometer that adds a temperature icon to the Windows taskbar. Based on DS1621.
http://www.riccibitti.com/pc_therm.htm
http://www.anotherurl.com/therm/schematic.htm