For tons of interesting info on area code expansion and history visit:
LincMad Telecom (includes area code lists)
or
AreaCode-Info.Com
Useful Links
Qwest DEX Find a Person On-Line
Qwest DEX Find a Business On-Line
International Country Codes and City Codes
Country and City Codes from AT&T
Need a map? University of Texas Perry-Castaneda Library Map Collection
United States Postal Service Zip Code Look-up
North American Numbering Plan Administration area code lists and maps.
Association of College and University Telecommunications Administrators
Federal Communications Commission
Colorado Public Utilities Commission