A really amazing list on MetaFilter, which I stumbled on entirely by accident. Beware; there are quite a few timesucks on this list 😉

A short, randomly chosen excerpt. Merely the tip of the iceberg:
PacMan Dossier.
Yarchive.
dedicated to the knitting of the authentic Doctor Who scarf.
RatBehavior.org.
Peacoat dating.
A history of Argentinian public transport tickets.
Alan Cooper’s All About Homonyms.
The Soda Can Library.
Internet Pinball Serial Number Database
a four-way listing of railway codes
Fire Lookouts in the Pacific Northwest
Alloy Artifacts
http://www.arcade-museum.com/
Taxonomic Data of the Breadties of the World
Rob’s Puzzle Page
The Guitar Rig Database
http://www.tapedeck.org/
Brand Name Pencils
Peeron LEGO inventories
221B Baker St
tommy westphall’s mind: A MULTIVERSE EXPLORED
Cosmic Baseball
Railway Telecomms
FreeCell — General Information and Index of Solutions

“What are some comprehensive one-topic websites maintained by cranky old guys (or gals)?”

The Old Man and the C Drive – internet websites cranks | Ask MetaFilter