Bookslut, a must-have site for all book lovers (tsk, no, it is not a naughty site), has a great page that lists some challenged books and some of the reasons why they were challenged. It also notes that for every reported challenge, there are 4 or 5 unreported challenges.
Another great resource is the Alibris site (it's like Chapters, but for independent new and used book sellers). They have a list (with reasons why) of banned children's books and a list of books that were banned for political reasons. They even have a list of frequently challenged authors (with links to their books).
Keep reading those banned books!