Folly Fanciers, I present the Nonument.
Strictly speaking, if your definition of a folly is a building with no particular function then this one does not qualify. On the other hand if it merely has to show folly in the designer or builder, then it does that in spades.
Commissioned by the City of The Hague as a surveillance hut for a bicycle park for the seaside resort of Schrevenige, the Nonument also acts as a piece of public art. The hut is contained in an object that is part monument, part strange fortification and part folly, deriving from seaside architecture, fortifications, lighthouses and earthworks. On top of the monument is a small house that periodically catches fire.
In January 2006 the project was featured on a series of postage stamps showcasing artworks by international artists in the Netherlands.
It was designed by the award-winning London practice FAT (Fashion Architecture Taste), run by Sean Griffiths, Charles Holland and Sam Jacob They have designed a number of interesting and eccentric buildings which seem particularly popular with the Dutch. Their web site is at http://www.fashionarchitecturetaste.com and has lots more examples of quirky modern design.