MQ-3

Sensitive for Alcohol, Ethanol, smoke
The heater uses 5V
The Arduino blog about the "breathalyzer" using a MQ-3 : http://arduino.cc/blog/2010/09/23/arduino-breathalyzer-calibrating-the-mq-3-alcohol-sensor/
The MQ303A (also on this page) is like this sensor, but uses a lower heater voltage.

Search for datasheet: http://duckduckgo.com/?q=%22mq-3%22+gas+sensor+filetype%3Apdf

source
http://playground.arduino.cc/Main/MQGasSensors