Online phone directories can be a useful tool in looking up someone's number or address, much like a traditional paper phone book. However, the online phone books also provide maps and directions to the address that the phone number is registered to plus they have a larger data base--as in not just your local area as well as toll-free numbers.
There's even reverse phone and address books. These are particularly popular with businesses to get an address or directions to a contact's location that they only have phone numbers for. The reverse look-ups get their information from publicly available information, meaning if your number is non-published, your number won't be given out. None of the data bases have cell phones in their directory. If you don't want your information made available on the web, contact the individual sites' managers.
For This Week in Geek, I'm Mark Avery.