Decayed Machinery, another group Livejournal. Lots of rusty cars, and some industrial sites.
Lapsed Modernist is the photoblog of an urban explorer. Not only is the work excellent, but she has access to some really nice locations.
The Unconscious Art of Demolition, a Flickr pool. A superb collection of mostly exquisitely textured and weathered walls.
Medianeras, another Flickr pool devoted to the remnants of demolished medianeras, dividing walls.
More at El arte de las medianeras.
50 Grams of Urban Loneliness, a Flickr photoset. These places are not really abandoned, but they are lonely. And beautifully photographed.
The Ghost Signs Pool. The past fading before your eyes. (I'm quite fond of the first one, by the way: "Blue Ribbon Tea".)
Opacity is the urban ruins photography of Motts. Very shiny and professional, great compositions, and many wonderful locations. Hard to choose samples -- I just about closed my eyes and clicked. And clicked and clicked.
Via Dream Tree.
The stunning photography of Shinichiro Kobayashi. Many excellent photos of gloriously decaying places from all over Japan.
Via the nonist.
I'd just like to add that many of these photos reminded me of places I've been in games -- I'm sure I've seen this room somewhere in Nova Prospekt in Half-Life 2, and this place certainly is from some corner or other of Riven's Boiler Island.
Undercity.org is the work of Steve Duncan, "a guerilla historian in Gotham". There's actually not that much history about the photos (stories and articles are in a separate section), but his beautiful shots of moody subterranean places, like old mausoleums and miles of subway tunnels, are testament enough.
Via the nonist.
Photographer David Maisel, has, among other records of decay and ruin (such as his "black maps", aerial photos of environmentally impacted landscapes, and the "library of dust" containing the cremated remains of asylum inmates), an excellent series of photos of the crumbling asylum itself.
Modern Ruins, photographic essays by Shaun O'Boyle. Numerous photo essays, each exploring a specific location running the gamut from hospitals and asylums to small dying towns to industrial sites.
Modern Ruins, the photography of Phillip Buehler. Many industrial and military subjects, like factories and airplane graveyards. His work includes a selection of panoramas presented in QTVR. He also has a small collection of Street Fossils.
Via the nonist.
Also, the nonist with some links on the subject of decay: notable among them are two articles by Brian Dillon, "Down in the Dump", in Frieze, and "Fragments from a History of Ruin" in Cabinet.
New Scientist invites us to "Imagine Earth without people": Left once more to its own devices, Nature would begin to reclaim the planet, as fields and pastures reverted to prairies and forest, the air and water cleansed themselves of pollutants, and roads and cities crumbled back to dust. (You can also read the article at Archinect, where the page design is more soothing.)
I'm not really a fan of this sort of anti-anthropocentric naturist creed, but it is poetic, at least.
Via Side Effects, a blog about decay.
This book sounds intriguing; this book, which I spotted and flipped through in the MoMA bookstore, is a beauty.