Beyond the already popular "archi-tourism", there appears to be a growing fascination with exploring places - particularly in cities - that are in ruin or where the public isn't allowed. Loosely coined Urban Exploration, it usually involves trespassing, not for vandalism and destruction, but for curiosity and most likely a rush. Zines like Infiltration illustrate the popularity and its fringe nature simultaneously.
The picture above, taken by Charles Janda inside the Manteno State Hospital, shows what unique discoveries can be made in these off-limit places. His page features an Urban Exploration/Ruins Gallery, links to other UE sites, and photo albums with recent photos, including the Manteno State Hospital (password: visitor).
This image of an abandoned natatorium is from a French page with a massive gallery of places in Europe, such as Bridges, Closed-down Buildings, Hospitals, Quarries and Tunnels. The moody photographs capture the feeling of time and unintended human interaction, what happens when buildings and other structures are abandoned and fall into disrepair.
Many other links can also be found at Chicago Urban Exploration, the Urban Exploration WebRing, and the Open Directory of Urban Speleology.
DO NOT TRY THIS AT HOME!
(Though I guess that's the point, since you can't anyways!)
Update 01.05.05: If you find the image of Manteno State Hospital appealing, check out The Manteno Project, "the most comprehensive source of information about the history of Manteno State Hospital. This site includes historical photographs, images, reports, and other information about the former Manteno State Hospital, operational in Manteno, Illinois from 1929 - 1985." Thanks to Michael A. for the link.