#1 on the 2005 Most Challenged list is It's Perfectly Normal, by Robie H. Harris. Here are the Top 10 challenged books, courtesy of the ALA. The press release also includes the reason for the book's challenges.
1. It's Perfectly Normal, "for homosexuality, nudity, sex education, religious viewpoint, abortion and being unsuited to age group"
2. Forever, by Judy Blume, "for sexual content and offensive language"
3. The Catcher in the Rye, by J.D. Salinger, "for sexual content, offensive language and being unsuited to age group"
4. The Chocolate War, by Robert Cormier, "for sexual content and offensive language"
5. Whale Talk, by Chris Crutcher, "for racism and offensive language"
6. Detour for Emmy, by Marilyn Reynolds, "for sexual content"
7. What My Mother Doesn't Know, by Sonya Sones, "for sexual content and being unsuited to age group"
8. Captain Underpants series, by Dav Pilkey, "for anti-family content, being unsuited to age group and violence"
9. Crazy Lady!, by Jane Leslie Conly, "for offensive language"
10. It's So Amazing! A Book about Eggs, Sperm, Birth, Babies, and Families, by Robie H. Harris, "for sex education and sexual content"
Thanks to the American Library Association for the poster image. You can even vote for your favorite banned book over at the ALA's web site.