I'm surprised Silent Garfield wasn't mentioned: http://www.silentgarfield.com/
Weird, Garfield minus Garfield has the same premise, and was mentioned. http://garfieldminusgarfield.net/ Wonder which came first?
This article is awesome, by the way. I was so addicted to dealing with garfield strips programmatically a few years ago. I wanted to work on a library which could analyze the images, reconstruct the layers and extract the text in order to do things such as removing specific characters, lines, randomizing extracting every single Jon/Garfield sprite and constructing animations out of it, etc.
I feel like Garfield is actually a very cool platform for this because there is a lot of data and not a lot of variations (garfield is extremely formulaic). Maybe this inspires someone to work on it.
(PS: Hint: If you are interested in working on it, scraping every garfield strip off the official website is extremely easy)