Police Car Web Site
Mostly US and Canada, with some interntional coverage. Also covers ambulances and fire engines.

Emergency Vehicles Online
UK and international emergency vehicles, but you have to pay a £5 annual subscription for full access.

Police Worldwide

111 Emergency
Police cars, ambulances and fire engines from New Zealand (111 is the emergency phone number there).

Not police cars, but worth mentioning:
Fire-engine-photos.com