Zeno’s Control Tower – World War II Aviation Links – a great resource, with links to many sites.
Cybermodeler Online’s Aircraft References – links for (seemingly) EVERY plane type.
wwiivehicles.com Squadron and Group codes – a table of squadron and group codes – with some incompletes.
Two related sites:
http://www.warbirdsresourcegroup.org/index.html – a great site to research particular planes.
http://warbirdinformationexchange.org/ – the forum that accompanies the site listed above.