She's perhaps best known for her Arthurian fantasy, Mists of Avalon, a retelling from the point of view of the women behind the throne. In my opinion, this novel is a worthy successor to Tennyson, Malory and T.H. White, and really essential reading for anyone interested in looking seriously at the Arthurian mythos. One of my most pleasurable academic experiences was the analysis of it that I did my freshman year.
I will always regret that I never met her. Her poor health in the last few years kept her from attending conventions, other than the small minicon she held in Berkeley. I could have gone to the last one, but I was very busy, and thought there would be time later.
At least I've been lucky enough to read her books.