According to Wikipedia,  A carbon monoxide detector or CO detector is a device that detects the presence of the carbon monoxide (CO) gas in order to prevent carbon monoxide poisoning. CO is a colorless and odorless compound produced by incomplete combustion. It is often referred to as the "silent killer" because it is virtually undetectable without using detection technology.

A new law requiring homes to be retro-fitted with carbon-monoxide (CO) detectors takes effect July 1, 2011.  All single-family residences which have fossil fuel burning appliances, fireplaces, or an attached garage are required to have a carbon monoxide (CO) detector on each floor.

The CO detectors may be battery or plug in with a battery backup.  Wal-Mart offers a a battery operated detector for $16.54 and a plug-in with battery back up for $33.54 

Unlike smoke detectors, CO detectors must be replaced every 7 years. 

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