I don't know of any really good site. The Army and USAF section of this site has sometimes been helpful, but it is a matter of luck if the person you seek is listed.
http://www.usofficerdocuments.com/index.html
You can do a google "site search" to find by a serial number.
E.g: site:
www.usofficerdocuments.com/ "1688073"
will find an officer whose serial number contains 1688073, the search will find:
YABLON, ABRAHAM O-1688073
(Actually, I just noticed that their built in "Search" works for serial numbers also.)
fold3.com can sometimes be helpful as names and serial numbers will appear in MACRs, sometimes just in the witness statements.