It's frightening to think how many websites I've registered with over the years, and how many of those sites I still use.  I can't remember the last time I tried to draw together all my active accounts, so here we go:

Livejournal
k00kaburra - my personal journal
fashion_piranha  - my book blog

Abandoned websites
Makaiju.net - I always intend to do something with it, but right now it's just a digital dust bunny.

Social Networking
Facebook - Indispensable.
Twitter - I still hate myself a little for getting one
(I probably still have a MySpace somewhere, but I don't really want to go looking for it.)

Specialized Twitter-esque Websites
Steepster - tea reviews
GetGlue - media-centric social networking; I use it to track my media consumption.

Book-related Sites
BookCrossing - book tracking website; I use it to keep track of what I read because otherwise, I have no idea.
BookMooch - trading website
LibraryThing - book reviews
Paperbackswap - trading website

I think that's everything...I hope that's everything!