Catholic Apologetics.info
This site contains a number of interesting articles worth reading whether or not one is needing to debate liberal "Catholics" or Protestants. Has many articles on Apologetics. Everything from Islam to Jehovah's Witnesses, to Mormons, Lutherans, liberal Catholics, non-denominationals, atheists, etc. A few historical articles as well.

ScriptureCatholic
A highly recommended resource, and one of the easiest to use, this site was started by a former Freemason, now traditional Catholic, John Salza. His site is excellent for pointing out Scriptural evidence for Catholic beliefs and even quotes the writings of the Early Church Fathers. A must see.

Our Lady's Warriors
Articles on the Faith, spiritual reading from books, Canon Law, etc.