Aside from Arthur himself, the figure most closely associated with Avalon is that of Morgan le Fay, identified by Geoffrey of Monmouth as the leader of the nine sisters who live there. While her name suggests that she was originally considered to be a fairy, later Arthurian mythology made her the daughter of Gorlois and Igraine, and hence Arthur's half-sister. I haven't done a whole lot of research on Morgan, but she fits into my general fondness for mythological women who don't take crap from the patriarchy, especially when they have magic to back themselves up. I wonder if she or Medea would win in a magical duel. Actually, a contest between witches wouldn't be a bad idea for a poll, although I'm not sure whether it should be a popularity contest or a battle of skill. Considering how most polls go, though, I'm sure different people would interpret it in different ways anyway. I also wonder whether Morgan would get along with Jenny Jump.
Figures 1-1 and 1-2: Pictures of Morgan le Fay
Moving on to some more modern media, Avalon was a significant part of the Disney cartoon show Gargoyles. In the show, it was the domain of the Fairy King Oberon, which fit with the cartoon's tendency to mix mythologies. Actually, though, the thirteenth century French epic of Huon of Bordeaux makes Oberon the son of Morgan and Julius Caesar, so the association certainly isn't totally out of left field.
And finally, if you'll cast your minds back to the late twelfth century, Avalon was then associated with Glastonbury, which is also an island (well, sort of; it was once surrounded by swamps) known for its apples. Monks at the abbey there claimed to have found Arthur's grave, and not only did this provide a significant boost in tourism, but also a way for the current monarchs to debunk the return of Arthur. "Look, it's his GRAVE. He's not coming back, because he's DEAD. So just settle down and be satisfied with the Plantagenets, capiche?" Well, actually, these were kings of French ancestry, so they were more likely to have said, "Comprendez?" But I think you get the idea.