Thought I'd start off a wiki about useful websites for camping trips...

http://www.campercontact.nl/en/ - Map of thousands of camp-sites across europe
http://www.eurocampings.co.uk/en/europe/ - ASCI site with inspected sites - saved us over £100 on our recent trip
http://www.ukcampsite.co.uk/ - lots of useful user reviews for campsites across the UK.
http://coolcamping.co.uk/ - more sites and reviews
http://en.airecampingcar.com/ - stopovers
https://www.searchforsites.co.uk/ - loads of campsites/stopovers with reviews