Please add links to other resources that you are aware of.

Encyclopedic resources: On-Line Encyclopedia of Integer Sequences | PlanetMath.org | Eric Weisstein's World of Mathematics | Wikipedia

Other online resources:

• arXiv.org e-Print archive
• Citeseer: Scientific Literature Digital Library
• Keith Kearnes, General Algebra and Lattice Theory Conferences, contact researcher, mailing list, bibliography,…
• MacTutor History of Mathematics archive Biographies of mathematicians, index of famous curves, …
• MathSciNet Mathematical reviews (requires subscription)
• RelMiCS: Relational Methods in Computer Science Conferences, tools, mailing list, bibliography,…
• WikiBooks Free open content textbooks and other classroom texts
• Zentralblatt MATH Reviews of mathematical papers (free queries for $\le 3$ reviews)

Books and surveys available online (see also http://www.ams.org/online_bks/online-books-web.html):

• Michael Barr and Charles Wells Triples, Toposes and Theories
• Wim Blok and Bjarni Jónsson Algebraic structures for logic (Lecture notes)
• Peter Burmeister Lecture Notes on Universal Algebra: Many-sorted partial algebras
• Peter Burmeister A Model Theoretic Oriented Approach to Partial Algebras
• Stanley Burris and H.P. Sankappanavar A Course in Universal Algebra
• Ralph Freese and Ralph McKenzie Commutator Theory for Congruence Modular Varieties
• Mai Gehrke and Bjarni Jónsson Bounded Distributive Lattice Expansions
• Rob Goldblatt Algebraic Polymodal Logic: A Survey
• David Hobby and Ralph McKenzie The structure of finite algebras
• George Hutchinson Lattices and Categories of Modules Lattice theory equations and interesting lattices
• Peter Jipsen and Henry Rose Varieties of Lattices
• Keith Kearnes and Emil Kiss The Shape of Congruence Lattices
• Olga Kharlampovich and Mark Sapir Algorithmic problems in varieties
• Don Monk An introduction to cylindric set algebras
• JB Nation Notes on Lattice Theory Chapters 1-6 Chapters 7-12
• Herbert Wilf Algorithms and Complexity