In my mind, fan art is unsolicited artwork submitted to a person or site that showcases the creator's talent while at the same time acknowledges the recipient's impact on their visit or own style.
I do not consider the following pieces "fan art" because 1. I asked the artist for a contribution and 2. These are artists with their own sites and projects who graciously took the time to fill in for me when I was unable to draw. Hence the "guest art" label.
Of course, these cartoons are property of the respective artists, not me...so please do not duplicate or transfer them without consent! Enjoy!