Garfield and LizThe comic itself perked up a lot when Jon ( the leading human ) finally got into a relationship with Liz, Garfield's veternarian. This changed a comic about a loser and his cat into a comic about a very fallible guy and his cat and the woman in their lives. Loser jokes get old, but relationship jokes can go on forever.
Garfield minus GarfieldSomeone noticed that since the cat Garfield doesn't actually speak, most of the strips have John acting very strangely. He airbrushed Garfield out of a few strips for a joke, started getting massive site hits. This grew into http://garfieldminusgarfield.net/ .
I read in the book that Jim Davis very good-humoredly blessed the project, which you have to admit is pretty awesome.
The Random Garfield generatorThe story I heard goes that someone thought it would be funny to put together three random Garfield panels. This one has to be used very carefully becauase of quite appropriate intellectual property concerns, but some will find it very amusing.
"...so this guy noticed that Garfield comics make just as much sense if you throw random panels together, and sometimes are actually pretty funny. He got a cease and desist letter. So he made the code available for people who wanted to try it for themselves. Here we go!"