How to find an unlisted phone number
You can search a pretty extensive database at ThinkDirectMarketing by name, by street address and by phone number.
Database America People Finder (Address or phone, or reverse phone number lookup)WhitePages.com lets you look up by name, by phone number and by street address.
Search Info USA by name and by phone number
Info Space for Phone and Address search and Reverse Lookup. In the US and Canada.
Primeris has a function called Fone Finder that looks up numbers in the US and Canada as well as International Switchboard
AnyWho (ATT) has Find a Person and Reverse Lookup.
Searchbug has Find by Name, Find by Address, Find by Phone and Area Code Decoder.
Phone Searches in the United Kingdom
AmeriCom Long Distance AREA DECODER Give them a number and they will tell you what city it's from in the USA or Canada. Canada 411
Yahoo has a list of resources
Whois/ White Pages / Directory Services
Telephone Directories On The Web
RingThem.com also has world directory listings.