Jacki was Spaghetti Cat in 2008. You don't have the time to make a better costume for this year, unless you have been working on it a while already. In case you've forgotten, here is the origin of Spaghetti Cat.
Brad O'Ferrell showed up at the "A Night To ReMEMEber" internet meme party in a Keyboard Cat costume. All it took was a cat costume and a keyboard.
Matt Cutts is a LOLcat. Or he was for Halloween last year. The cheeseburger was a nice touch!
This LOLcat costume is perfect for trick-or-treating. You won't get the full effect until you see the back of the t-shirt.
This Ceiling Cat costume should be easy to do, depending on the size of the ceiling. If you are going to sit on the porch and hand out candy, this would be OK. It might be a problem if you are a cab driver.
You can buy Hello Kitty costumes of all kinds, but making your own is a real accomplishment. Pink Daisy at Craftster made her own version of Hello Kitty for Halloween 2008.
This costume was posted earlier on Neatorama, but this collection wouldn't be complete without Lime Cat, Melon Cat, Helmet Cat, or whatever you want to call this. Flicker user Alida Saxon’s brother put this costume together at the last minute. The helmet is a basketball painted green, and the fur was salvaged from a stuffed toy.
A warning: if you dress as an internet meme for Halloween, be prepared to explain yourself. Unless you're meeting up with your online group, you'll probably have to tell the whole story of your costume. If you're willing to do that, the few who will "get it" will make the entire effort worthwhile.