The IHC 185 has a excellent reference section, mostly devoted to American made pocket and wrist watches.
There are many links to specific sites scattered within the posts. Here are a few:
Hamilton serial numbers
Elgin serial numbers
Waltham serial numbers
There are several other links located at the NAWCC Chapter 149 website at this address:
Omega has an "official" serial number table here:
Here is a an "unofficial" table with a lot more information, posted by Ryan Rooney:
The best site that I've found for research on vintage Rolex is here:
Here is the link for Rolex serial numbers:
I just found a real nice Longines information site:
Serial number lookup is here:
I have several other sites that I can't recall right now.
This should be a good start though!
Hope this helps,