This looks pretty interesting. Just got this info from Parallax PR:

The CO Gas Sensor Module is designed to allow a microcontroller to determine when a preset CO gas level has been reached or exceeded. Interfacing with the sensor module is done through a 4-pin SIP header and requires two I/O pins from the host microcontroller. The sensor module is mainly intended to provide a means of comparing carbon monoxide sources and being able to set an alarm limit when the source becomes excessive.

Uses the MQ-7 CO Gas Sensor
Easy SIP interface
Compatible with most microcontrollers

For details, visit Parallax and search on “27931.” The module is priced at $30.