Spare clothing
...appropriate clothing (inc. hat, gloves, etc.) for being outside of your heated car (or in it when it's not running...)
Snow boots/socks for your tyres, or
Snow mats to put underneath them (although someone did point out how you keep them/retrieve them once you've got yourself going..!)
De-icer
Scraper/squeegee for your windows
An old sheet to keep in the boot - to put over the windscreen in the evening when frost is forecast so you can remove it (and the ice) in the morning - Voila. No scraping...
The latest copy of
Advanced Driving....
In reality - I'll probably have very little of this and the other stuff mentioned with me. If the weather's that bad, I'll stop at home...