Official Ecclesial Sites

The Holy See (Vatican)
The United States Conference of Catholic Bishops

Magisterial Teaching and Basic Reference

Bible (New American translation)
Bible (Revised Standard Version Catholic edition)
Bible (New Jerusalem translation)
Catechism of the Catholic Church
Documents of Vatican II Ecumenical Council
Ecumenical Councils of the Catholic Church
Historical Catholic Catechisms
Writings of the Church Fathers at New Advent
Patron Saints Index
Code of Canon Law (Latin Church only)
Code of Canons of the Eastern Churches

Catholic Apologetics

Called to Communion
Catholic Answers
St. Paul Center for Biblical Theology (Scott Hahn)
Biblical Evidence for Catholicism (Dave Armstrong)
Nazareth Resource Library (Jimmy Akin)
Corunum (Joe Gallegos)
Bible Christian Society (John Martignoni)
Defenders of the Catholic Faith (Stephen K. Ray)
Coming Home Network (former non-Catholic pastors & ministers)

Other Catholic Organizations

Theology of the Body Institute / Christopher West
Association of Hebrew Catholics
EWTN Document Library

Some Classic Catholic Books Online

On the Incarnation of the Word of God by St. Athanasius
An Essay on the Development of Christian Doctrine by John Cardinal Newman
The Catholic Church & Conversion by G.K. Chesterton
Orthodoxy by G.K. Chesterton
The Spirit of Catholicism by Karl Adam
Theology and Sanity by Frank Sheed
The Eastern Christian Churches by Ronald Roberson
Summa Theologica by St. Thomas Aquinas
Church of the Word Incarnate by Charles Cardinal Journet
The Spirit of the Liturgy by Romano Guardini
Liturgy & Contemplation by Jacques & Raissa Maritain
Roots of the Reformation by Karl Adam
The Dumb Ox by G.K. Chesterton
Faith of Our Fathers by James Cardinal Gibbons
Where We Got the Bible: Our Debt to the Catholic Church by Henry Graham
Practice of the Presence of God by Brother Lawrence
The Man Who Was Thursday by G.K. Chesterton

Other Recommended Reading

Protestant Intro to Catholicism
Essential Catholic Library
Scripture, Tradition, & Church for Protestant Inquirers
Eucharist, Liturgy, & Sacraments for Protestant Inquirers
Mary & the Communion of Saints
Reading Scripture as a Catholic
Revelation & End Times: Catholic Perspectives
Deeper Into Catholic Theology (for beginners)
Catholic Conversion Stories
Answering Anti-Catholic Bigotry
Great Christian Literature
Toward Christian Economics 1
Toward Christian Economics 2