The following are some links that I find useful:

General web pages

UKRS (uk.rec.scuba) homepage - Keith Lawrence's excellent UK diving website
Scuba UK (The Drogon Network) - The Scuba-UK mailing list web page
GAS diving UK homepage - Good kit configuration info
UK Diving homepage - the Dive magazine website
Jason's Home Pages - another well known UKRS'er
Greg Roach's homepage - large amount of useful links here
Drogon Network - Gordon Henderson's scuba page with links to Scuba-UK archives
Suunto PC interface - Dave Cordes' Suunto Solution Alpha PC interface construction
Hogarthian kit rigging - Todd Leonard's pages on setting up a hog rigged twinset
Trimix - stuff all about - you've guessed it - trimix
Wah Wah article - or why you don't want to dive deep on air...
Deploying a DSMB - Matt Steven's excellent article on DSMB deployment

Weather related sites

Sea Temperatures around the UK - Find out how warm it is...
Beaufort wind scale and wave height
Met Office weather page - The latest weather reports
ECMWF - weather charts for 3-6 days ahead

Wreck Information

Ship Losses around Britain - List of some of the wrecks around these shores

Dive Site information in and around the UK

Map of Dorothea Quarry
UK Visibility reports - Greg Roach's UK Visibility report web page
Dorset Underwater - Andy Carrington's site of Dorset diving info

Dive manufacturers, dive equipment and dive shops

Buddy Inspiration - closed circuit rebreather
Mike's Waterfront Warehouse - dive shops in several locations
SDS Watersports - large retail outlet in Sheffield
Runnymede Dive - numbers for both Reading and Runnymede
Wittering Divers - South coast
Custom Divers - wings and torches
The Scuba Centre - Run by Izzy and Nina
Aquarius Diving - Didcot
Breakwater Dive Centre - Portland

Dive Organisations

BSAC - the British Sub Aqua Club
PADI - the Professional Association of Diving Instructors
SAA - the Sub Aqua Association
IANTD - International Association of Nitrox & Technical Divers
IADI - Independent Association of Diving instructors
GUE - Global Underwater Explorers

Dive Clubs

Reading BSAC - my local club