Fone Finder gives you information about area codes, exchanges, and country codes.
THE List of Internet Service Providers.
The United States Postal Service provides all the details you need for sending mail (the old-fashioned way), including the zip code for any address you enter.
The UPS site lets you track packages and price deliveries.
InfoSpace has a very useful and unique feature: regional/metro search, which searches the entire metro area of the city you specify. It gives a count of matches, but doesn't display the full listings until you click on one individually (it shows just the name, city, and state otherwise). It also sorts by state (if you didn't search for a particular one), rather than alphabetically. Another unique feature: you can do reverse searches based on phone number (home or office), area code, and even address!
Switchboard does NOT give a count of matches (very annoying), but displays the full listings. Source: Database America
AnyWho allows more search options, and displays full listings. However, it does NOT give a count of matches (very annoying). It also sorts by state (if you didn't search for a particular one), rather than alphabetically.
infoUSA requires that you enter a state for the address. It allows reverse searches by phone number and gives a count of matches. I don't know what its source is, but it seems to be missing a lot of people!
SuperPages, by Verizon, is the best way to find businesses. It's an informative yellow pages for the U.S. that lets you narrow your search by name, category, zip, area code, location on a map, etc. It gives details about each business and locates it on a street map for you. Source: Acxiom. Was BigBook
Switchboard requires category, city, and state to be entered.
The Internet 800 Directory is a nationwide 800/888 phone directory.
Yahoo! People Search
The Internet Address Finder does a pretty good job, and I don't know how it gets its addresses.
The Internet Public Library redefines "browsing through the library".
Encyclopedia.com provides - what else - encyclopedia lookups.
Foreign language references:
The Official U.S. Time is provided by the US Naval Observatory (USNO) and the National Institute of Standards and Technology (NIST).
Read all about Daylight Saving Time. Remember, it's from the first Sunday in April to the last Sunday in October (for the US (mostly)).
Time and Date.com provides information about time zones, daylight savings time, and calendars.
NIST also provides official Unit Conversion tables for American and metric measures.