192.com: For those who remember it 192 is the old directory enquiries number for the UK. 192 lists address and telephone information from telephone directories and the electoral roll among other sources.
BT Directory Enquiries: this service allows you to search by person's name or location, right down to house number. Also allows you to store number in online address book. 10 free searches a day and up to 200 a month after which you have to register and pay for the service.
BT STD codes: as well as listing the local std codes and having a search engine. This site also has some extra functionality, if you key in your std code it shows you which other codes you can dial and still pay local rate
Infobel: personal and business telephone directory - free search facility.
international dialling codes: dialling codes for all countries from CountryCode.org and how to use them, also includes a database search functionality so you can check which country missed calls came from.
Irish yellow pages: search by business type, by area or by business name. Covers Ireland.
Scoot business finder: search for phone number by business type, business name or geographical location.
Thomson Directories: well-known uk local business telephonedirectory. Website where you can search for businesses by name, area, business type.
ukphonebook: telephone directory containing phonenumber and postcode search engine. You must register and pay to use this service.
Yell.com: online home of the yellow pages business telephone directory