Ask Alix
Ask Alix has more than a million UK businesses included in its own online database as well as 22 million entries for businesses in 14 European countries. Site also functions as a general purpose search engine.

BT
Find telephone numbers for businesses and people free of charge, has total coverage accross the UK

ThomWeb
From the company behind the famous Thompson Local directory. Search for businesses and services accross the UK.

YELL.COM
Online version of the well established Yellow Pages business directory. Can search by entering keywords or allows browsing by category.

Kelly Search
Specialist business directory covering industrial products and business services. Site has a keyword search facility which is very useful for sourcing particular products, supplies and services as well as searching by business names and classifications.