AnyWho
An excellent phone directory has search tools for people, businesses, and reverse lookup (by phone, not address). Reverse lookup can be useful for finding multiple household member names.
SwitchBoard
Similar (if not identical) in coverage to AnyWho, but does allow for address-based reverse lookup
USPS WWW Server
Get your Zip + 4 codes right here! Those wonderful public servants at the United States Postal Service have put together a server that dispenses zip codes for specific addresses, postal rates, and other USPS info. If you need to find a company address, enter the company name and the city. The server will return the address with full zip.
Jay Computer Services Zip and Area Code Lookup
Want to know all the zip codes within 100 miles of a particular spot? This is the place to go. Want to know all the zips within a city, county or state? JCS will tell you. They will also tell you all the area code changes in any region.