It's The American Library Association's Banned Books Week: Celebrating the Freedom to Read. September 25−October 2, 2010.
According to the ALA, they've tabulated statistics on banned books for only the last two decades.
The Top-10 from 1990 to 1999 (Feel free to check the Full List.)
1 Scary Stories (Series), by Alvin Schwartz
2 Daddy’s Roommate, by Michael Willhoite
3 I Know Why the Caged Bird Sings, by Maya Angelou
4 The Chocolate War, by Robert Cormier
5 The Adventures of Huckleberry Finn, by Mark Twain
6 Of Mice and Men, by John Steinbeck
7 Forever, by Judy Blume
8 Bridge to Terabithia, by Katherine Paterson
9 Heather Has Two Mommies, by Leslea Newman
10 The Catcher in the Rye, by J.D. Salinger
The Top 10 for 2000-2009 (And the full list again)
1 Harry Potter (series), by J.K. Rowling
2 Alice series, by Phyllis Reynolds Naylor
3 The Chocolate War, by Robert Cormier
4 And Tango Makes Three, by Justin Richardson/Peter Parnell
5 Of Mice and Men, by John Steinbeck
6 I Know Why the Caged Bird Sings, by Maya Angelou
7 Scary Stories (series), by Alvin Schwartz
8 His Dark Materials (series), by Philip Pullman
9 TTYL; TTFN; L8R, G8R (series), by Myracle, Lauren
10 The Perks of Being a Wallflower, by Stephen Chbosky
Also worth check out is the Radcliffe Publishing Course Top 100 Novels of the 20th Century,
which includes 46 banned books, with all of the top-9 listed as challenged.
The ALA also tabulates statistics ranging from yearly challenges to most cited reasons for a ban.
(Interestingly, the 2nd most common reason is "Unsuited to Age Group.")
Finally, Wikipedia also maintains their own list,
though it has two major accuracy warnings plastered to the top of the page (I only cite it because I figure it's inevitable anyway).