The Holy See- All major Church documents, papal writings, the Catechism of the Catholic Church, art and even "secret" archives can be found here. The search engine is a bit cumbersome, and not all documents are available in English, but it's still the first place for all things Catholic.
U. S. Conference of Catholic Bishops - The official site of the U. S. Catholic bishops includes an online Bible, the Catechism, publications, movie reviews and more.
Knights of Columbus- Home of the world's foremost Catholic fraternal benefit society.