Spencer Rescue Lifehammer: http://www.spservices.co.uk/product_info.php/cPath/23/products_id/40

Fitted in my car. Bascially it is a regular hammer + Window breaking hammer (i.e a hammer with a steel point that will smash easily through any car window glass) + a seat belt cutter (a safety razor sharp blade that can cut through thick material but with no risk of cutting yourself on it).