A while back I came across Answering Islam, a website for Muslims who want to learn about Christianity, as well as for Christians who want to better understand Islam in order to refute it from a biblical standpoint. I've only gone through a few of the categories and articles, so read with discernment, but so far this seems like an excellent resource, especially if you are building relationships with Muslims through work, school, evangelism efforts, etc.
A few sections of note:
Why They Converted - Personal testimonies of Muslims who came to saving faith in Christ.
Muslims Asking Christians - Answers to common questions.
Muhammad - Examining "whether Muhammad is a legitimate / true prophet in the tradition of the Biblical revelation." (obviously he is not!)
I realize this might not interest a lot of you, but again, it might be a helpful site to bookmark if you ever need some help in discussing Christianity with Muslims. Keeping in mind, as with any study of cults and false religions, that the best way to spot error is to have a firm knowledge of truth. (Found in the Word of God, the Bible!) I once heard it put like this: To know if a stick is crooked, you only need to hold it up to a straight one!