I have reread The Outlander series by Diana Gabaldon several times as well. Outlander is the of two books that I reread as soon as I'd finished it. Atlas Shrugged was the other.
There are several other books, usually series, that I have reread several times over the years. Some of them, like The Belgariad and the Malloron, by David Eddings, I used to read almost annually for about twenty years.
The Incarnations of Immortality series by Piers Anthony is another that is eminently re-readable.
I almost always pick up on new things when I reread a book - foreshadowing I didn't notice before and other small details.
What books do you love to read over and over?