It seems there is no end to the number of amusing cat sites on the internet. Of course, I've already mentioned catenema.com, but there are many more.
At Rate My Kitten, users post pictures of their cats and rate other users' cats. Since I don't own a cat, I post pictures of my neighbors' cats. This site is a lot less demeaning than HOT or NOT?, upon which it is based, and a whole lot less disgusting than Rate My Poo.
Why Cats Paint attempts to answer that very question. Be sure to view the videos, and if you're really interested, there is a book.
The Cats' House details the work of a couple to convert their entire house into a cat paradise for their many cats. Unfortunately, their site is currently offline, but you can check out their books at Amazon -- The Cats' House and Cats into Everything.
At My Cat Hates You you can spend all day looking at photos of cats in really bad moods.
Be sure to bookmark Big Dave's Kitty Litter Cam and check it frequently. If you're lucky, you may get to see a cat pooping, and if you're exceptionally lucky, you'll see the owner's dog eating cat poop straight from the litter. Also, check out Big Dave's recipe for kitty litter cake.
Finally, you're not going to want to miss Bonsai Kitten, but, unfortunately, you may have to, since their site is currently down. You can see the cached page here, minus the pictures. And you can read about the controversy surrounding the site here and here. (Don't worry; people do not actually make bonsai kittens.)