Abandoned Britain - Photographs of disused properties, mostly asylums so far.
Abandoned UK - A startlingly poorly designed site featuring pictures of the Saro Products factory.
AGC UE - Chronicles of culvert exploration in Northern England.
Canadian Red Cross Memorial Hospital - A thorough site devoted to Berkshire's abandoned Canadian Red Cross Memorial Hospital, including a virtual tour.
Cane Hill Mental Hospital - A site devoted to an enormous abandoned asylum in London.
Derelict Sensation - The site of a group of that fetishizes all things abandoned (or "derelict"... English people are so cute).
Disused Stations on London's Underground - A well-illustrated guide by a non-explorer, featuring lots of fascinating history.
Friends of Williamson's Tunnels - Fan site for an odd collection of tunnels located under Liverpool.
Hellingly Asylum - A history and many pages of photos of an abandoned asylum in East Sussex.
Hidden Glasgow - A well-designed and often updated guide to places to climb on and in around Glasgow.
Nettleden - The site of a group interested in all things secret, featuring expeditions, news, forums and a chat service.
Offkilter - Abandoned buildings, mostly hospitals so far, around Cambridge.
Project Z - A London-based site devoted to hijinks generally.
Scotland's Secret Bunker - A commercial site promoting paid tours of a bunker near St. Andrews.
Secret London - Somewhat academic look at hidden underground sites, mostly Cold War leftovers, in the greater London area (currently down, hopefully temporarily).
Sub-Urban - Good-looking site focussing primarily on underground attractions, with some useful advice about mapping and equipping.
Subterranea Britannica - Large group interested in man-made underground structures; some members are friendly to urban exploration, some are not.
Truro's Forgotten Hospital - Take an online walking tour around the facilities of this abandoned hospital from the early 20th century.
Underground Kent - Ex-military tunnels and fortifications in Kent.
Urban Exploration Galleries - Cemeteries, asylums, hospitals, rooftops and more in the area around Glasgow.
Urban Underworld - This site is a smaller, more amateurish effort than the Subterranea Britannica site, but it is also friendly to urban exploration.
Urbanex - A flash-only site about urban exploration in the UK.
Urbex - A vaguely bloggy urban exploration related site, featuring write-ups of three or four sites and a message board.
Urbex UK - This site is different from the ones called "Urbex", though all are in the UK.
White Cliffs Underground - Exploration of ex-military tunnels, fortifications and underground works in Kent.
Worldoftheshadows - Photographs of disused properties, such as hospitals, factories and bunkers.
Yes Men - The Yes Men impersonate organizations, crash conferences and blur the lines between the official and the un. A great site.