Some useful cartoon sites

Cartoonist Club of Great Britain
Cartoonists
Portfolio of over 100 different cartoonists
Cartoon Stock
UK Cartoonist directory with samples and links

US Sites:

United Features
USA syndication featuring strips and cartoons and with how to guides on submitting cartoons, drawing etc. Includes links to other sites.
King Features
As above
Universal Press Syndicate
As above
Don Markstein's Toonopedia
General cartoon site with information on the US cartoon market with plenty of samples and FAQs