Many of us have used Joe Baughers site for US military serial numbers. http://users.rcn.com/jeremy.k/serialSearch.html I got to looking for serials for other countries and figured this could be a help to modders in SF2 and DCS
RAF current aircraft
Former RAF aircraft (gallery, not a listing like others)
French Air Force (does not give a listing but explains how the system goes)
Luftwaffe (post 1956)
this site needs some hunting, but many of the air arms that can be looked up give information on how their air craft are serialed
Hope this can help modders of all stripes and please feel free to add to this list.