ninebelow has posted a link to the Guardian's list of the twenty most re-read books and in 
instant_fanzine a poll about which books you have read more than once.1. The Harry Potter series - JK Rowling
2. The Lord of the Rings - JRR Tolkien
3. Pride and Prejudice - Jane Austen
4. The Hobbit - JRR Tolkien
5. Jane Eyre - Charlotte Bronte
6. 1984 - George Orwell
7. The Da Vinci Code - Dan Brown
8. The Lion, the Witch and the Wardrobe - CS Lewis
9. Wuthering Heights - Emily Brontë
10. Catch 22 - Joseph Heller
11. Notes From a Small Island - Bill Bryson
12. To Kill a Mockingbird - Harper Lee
13. Flowers in the Attic - Virginia Andrews
14. Black Beauty - Anna Sewell
15. Good Omens - Neil Gaiman and Terry Pratchett
16. The Bible
17. The Hitchhiker's Guide to the Galaxy - Douglas Adams
18. Bridget Jones's Diary - Helen Fielding
19. Gone With the Wind - Margaret Mitchell
20. Great Expectations - Charles Dickens
I've read LotR, Pride and Prejudice, Jane Eyre, 1984, Lion the Witch and the Wardrobe, Wuthering Heights, and Hitchhiker more than once. I've reread 'Half Blood Prince' from the HP series, to get me ready for the Deathly Hallows, and it stood up quite well I thought.
I can re-read a few authors any number of times: Jane Austen, Dashiell Hammett, Douglas Adams, HP Lovecraft. They just never seem to get old - four quite different writers, but they just don't irritate me at all. I read them when I'm bored with everything else.