The Èulogos project for a qualified, highly accessible, multilingual, open access and XML-based IntraText Digital Library
The website of Adoremus: Society for the Renewal of the Sacred Liturgy.
On Universalis, you can get a calendar, a lot of parts of the Missal and the Liturgy of the Hours, but due to copyrights issues, not everything is free.
Texts of the Roman Breviary following the rubrics and instructions for the recitation of the Divine Liturgy as set forth in the Bull Divino Afflatu of Pope St. Pius X in November 1911. We include these in our website because a lot of texts of the Breviary currently in use are identical to the texts of the older Breviary provided by this website. If you copy texts from this website for using these in liturgical celebrations according to the new rules, be aware that the letter « j » is no longer used in the current Vatican editions, but that the letter « i » is used instead of it.
The Circular Letter Concerning the Preparation and Celebration of the Easter Feasts from the Congregation for Divine Worship and the Discipline of the Sacraments (January 16, 1988)
The Instruction Liturgiam authenticam on the use of Vernacular Languages in the Publication of the Books of the Roman Liturgy from the Congregation for Divine Worship and the Discipline of the Sacraments (April 25, 2001)
For scores and PDF liturgical books, the website www.musicasacra.com provides an amazing collection for Gregorian chant and Latin liturgy.
Steven van Roode has put a very complete collection of booklets online for singing the divine office in Gregorian chant according to the forma ordinaria.
The Caecilia Project hosts an extended database of Gregorian PDF scores and the GABC sources used to typeset these.
The Gregobase site provides a fast growing interactive database of GABC scores.
The Lectionary website of Father Felix Just S.J. provides comprehensive tables of the scripture readings from several Latin and English editions of the Catholic Lectionary for Mass, so that you can easily obtain an overview of all the readings used in a particular liturgical season (Advent, Christmas, Lent, Easter) and/or in a particular year of Ordinary Time (Sundays in Years A, B, C; Weekdays in Years I & II). Cross-references and comparative analyses are also provided to enable you more easily to see the differences between various editions of the Lectionary for Mass.