The Vatican Website contains access to Vatican news, encyclicals, apostolic exhortations, etc.
The website of the United States Conference of Catholic Bishops offers access to news, documents of the USCCB, ministry resources, daily Mass readings, the New American Bible, the Catechism of the Catholic Church, etc.
New Advent website provides some news, the Catholic Encyclopedia, the Summa Theologica, writings of the Church Fathers, and the Bible.
The New Liturgical Movement is an interdisciplinary website on liturgical issues.
Christian Classics Ethereal Library offers online access to the Christian classics.