The Catholic Faith in General
A great website with multiple resources on the Catholic Bible, the Church Fathers, the Summa Theologica of St. Thomas Aquinas, the Catholic Encyclopedia (1912), and a good collection of Church and Papal documents. Also contains Church News on the front page.
The 255 Infallibly Defined Dogmas and 102 Certain Truths of the Catholic Faith. This site contains the complete list!
RCIA outlines explaining many areas of the Catholic Faith in a systematic manner. This is not just for converts to Catholicism, but contains many teachings that most Catholics would do well to recall (or to learn for the first time!).
Online Catholic E-Books
This website contains the traditional Douay-Rheims English Catholic translation of the Holy Bible. An interlinear with the Latin Vulgate is also contained on the site, as well as the well-known commentary by Bishop Challoner (1749-1752).
St. Patrick's Basilica in downtown Ottawa has a good online resource with several Catholic e-books that are in the public domain (out of copyright). The books can be read online or downloaded, and most of them are regarded as Catholic classics.
This is the text of the classic Baltimore Catechism with accompanying notes and explanations for teachers so that they can explain the catechism to students as they teach it. Very helpful for understanding the Catholic Faith!
This Catechism was designed to help Parish Priests pass on the Faith to those whom they are teaching. It is a great Catechism for any Catholic and can really deepen one's knowledge of the Faith.
This is the modern universal Catechism published under the authority of Pope John Paul II. It does not use the classic question-and-answer format of all other Catechisms but instead explains the Faith in a narrative manner.
On the Sacraments
Undoubtedly one of the best, if not the best, website on all things pertaining to the Holy Eucharist: from Communion, to Adoration, to Doctrine, to Eucharistic miracles, and much, much more!