You know, for some strange reason I don't think I ever saw this thread before. Weird.
Anyway... My first answer is: Anything written by Douglas Adams. This includes, but is not limited to, The Hitchhiker's Guide to the Galaxy series. There are only five novels and a short story, as far as I am concerned. The so-called "continuation" from that does not really exist, at least not in any canonical way. I read all his stuff about once every 1.5 years, these days.
I have read Jurassic Park, by Michael Crichton, several times, for some reason. It's the only Crichton novel that seems to (mostly) hold up well, for its age.
I do re-read non-fiction science books, every now and then, but I don't think I can single out any of them out, right now.
I have read The Hobbit and the Lord of the Rings trilogy a few times, though it might be a while before I ever get to them, again. Perhaps I will do so right before The Hobbit movie comes out, assuming it ever does.
The only other novels I read more than twice, as far as I can remember, are the Myst series, based on the computer games.
A few classic novels I have read twice are 1984, by George Orwell; and Gulliver's Travels by Jonathon Swift.
And, I read a few Star Wars novels, especially the original Thrawn trilogy by Timothy Zahn, twice.