I've plowed my way through the Honor Harrington novels by David Weber a couple times, as well as Steven Erikson's Malazan Book of the Fallen series, and the Hawk & Fisher and Nightside books by Simon R. Green... Usually every five years or so, I get the urge to go back through each of them.
Watchmen gets read at least every two years, as does V for Vendetta.
There aren't really that many things I reread more than once or twice, though. Usually by then I have the whole story stuck in my head and don't really need the books anymore - I can just play it in my head like a movie.
I've read most of the Dresden Files books twice.
The Hobbit and LotR have both been read about three times. Seems to be a ten-year thing.
The three main Dragonlance novels three times.
J. Michael Straczinski's Rising Stars novels three times.
Most of Nancy A. Collins' Sonja Blue novels four times.
Sergei Lukyanenko's Nightwatch series three times.
Lillith Saintcrow's Bannon & Clare novels three times.
And Austin Grossman's novel Soon I Will Be Invincible has been read three times, although I forsee rereading it again at least once more - it's just frackin' hysterical.