"There's a lid for every pot," the old saying goes, and the Internet sure allows the oddest-shaped lids and pots to find one another and discuss lid-pot relations at great length. Many of the communities discussed here will probably be old hat to you by now (Second Life? please) but then there are the ice chewers -- and the "boytaurs." Not for the faint of heart!
Here's another list of unusual social networks that may be more your speed -- if you're into hamsters or farmers or zombies.