www.cinematography.com
One of the best cinematography forums available. There is always a huge amount of very good information available.

www.cinematography.net
Ran by British DP Geoff Boyle,this is another cinematography list where you can find answers to pretty much everything.

www.theasc.com
The almighty one!! The most succesful cinematographers belong to this society.

www.bscine.com
British version of the above.

www.cameraguild.com
The International Cinematographers Guild.

www.imago.org
The European Federation of Cinematographers. A great site with the latest news.

www.gbct.org
The Guild of British Camera Technicians

www.imdb.com
We all know about this one...

www.boardsmag.com
If you are into commercials, you have to check this one out.

www.kodak.com
They have nice resources and the cinematography section is very good.

www.cameraspeed.co.uk
Film and TV equipment hire in the UK.
Best service and prices around.