For 2003, the most challenged books were:
1. Alice series, for sexual content, using offensive language, and being unsuited to age group.
2. Harry Potter series, for its focus on wizardry and magic.
3. "Of Mice and Men" by John Steinbeck, for using offensive language.
4. "Arming America: The Origins of a National Gun Culture" by Michael A. Bellesiles, for inaccuracy.
5. "Fallen Angels" by Walter Dean Myers, for racism, sexual content, offensive language, drugs and violence.
6. "Go Ask Alice" by Anonymous, for drugs.
7. "It's Perfectly Normal" by Robie Harris, for homosexuality, nudity, sexual content and sex education.
8. "We All Fall Down" by Robert Cormier, for offensive language and sexual content.
9. "King and King" by Linda de Haan, for homosexuality.
10. "Bridge to Terabithia" by Katherine Paterson, for offensive language and occult/satanism.
The 100 Most Frequently Challenged Books for 1990-2000 list is also on the ALA site.
I think what irks me the most is that most of these books were challenged in schools and school libraries. If someone doesn't want their kid reading a particular book, fine by me. I highly resent them deciding that my kid can't read it, though. That offends me more than the content of any book possibly could.