Adventure City - Scenery under the surface in the Netherlands and environs, often featuring great videos (in English).
Diggelfjoer - Abandoned buildings in and around the old industrial town of Enschede, Netherlands (in English; currently down, hopefully temporarily).
Disused Railways and Urban Decay - Photos and text relating to abandoned rail lines and rail buildings in the Netherlands (in Dutch).
Urban Adventure in Rotterdam - Petr Kazil's excellent and extensive page on urban art, buildering and exploration in Rotterdam and beyond (in English).
Urbaniaanse Mannem - Fantastic photographs and brief descriptions of expeditions to a variety of abandoned sites around the Netherlands, including factories, hospitals and seminaries (in English).