It occurs to me that this thread hasn't been updated for a while.
These are a few photo archives that I look at regularly :-
The good old IWM (but perhaps not everybody has seen it)
Welcome the Imperial War Museum's Collections Home Page
The Canadian Archives (lots of pictures taken in the UK and Europe).
Archives Search - Library and Archives Canada
Stilltime - No specific WW2 section but lots for vehicle enthusiasts and the wartime shots crop up in odd places.
Stilltime.net Stilltime.net
British Pathé - Not the easiest site but wonderful moving images, some with sound including Bren Carriers gunning their V-8s and the possibility to view stills at 1 second intervals. Lots of Mr. Cholmondley-Warner moments.
British Pathe Limited
Rich.