Opacity is on of the biggest urban exploration websites.
www.opacity.us
US based Urban Explorers.
www.urbanexplorers.net
Talk Urbex covers everything from very knowledgeable exploration's and a handy forum
www.talkurbex.com
Danvers State Hospital.
www.danversstateinsaneasylum.com
28 Days later is a UK based UE forum.
www.28dayslater.co.uk
Infiltration is one of the first UE websites.
www.infiltration.org
Urban-exploration is a French based site.
www.urban-exploration.com
Dark passage is a American based site & has been
around since 98.
www.darkpassage.com
Other great sites,
Abandonded Places
Weburbanist
Urban exploration resource website