Dallas Semiconductor 1-Wire™ digital thermometer sensor. Measures temperatures from -55 to +125 Celsius (-67 to +257 Fahrenheit) in 0.5 degree increments (higher resolutions can be obtained using a simple algorithm). The temperature is read as a 9-bit digital value, normally using two wires (ground and data) with data line powering the sensor (no standby power is required). The sensor is packaged either in an 16-pin SSOP or a three-terminal PR35 plastic transistor case. The latter is handy for most applications as it requires minimal wiring and space. The sensor should however be protected from moisture as it may cause readout inaccuracies. The 1-Wire™ bus is simple to implement (5 components needed for the simplest PC serial adapter) and can be used to connect several sensors and iButton units in parallel. Each sensor has a unique serial number which can be used to get individual readouts.
More information and basic PC adapter schematics can be found at Dallas Semiconductor web site. Two useful documents are the DS1820 data sheet and the DS Application Note #74. The URL:
http://www.dalsemi.com/datasheets/
There are loads of programs written for the sensor, for different platforms. Among the best is DigiTemp written by Brian C. Lane. Sources, instructions and binaries for Linux, DOS and Windows are available at
http://www.brianlane.com/.