Frege's Theorem
   Article in the Stanford Encylcopaedia of Philosophy, entitled `Frege's Logic, Theorem, and Foundations for Arithmetic'. Discusses the 1980 discovery of Crispin Wright that Frege's work of the Grundgesetze can be carried out in second-order logic with Hum
   http://plato.stanford.edu/entries/frege-logic/index.html

Humboldt State University: Argumentation and Critical Thinking Tutorial
   Jay VerLinden's resource offering reviews and tests on important concepts in formal logic.
   http://www.humboldt.edu/~act/HTML/

Logic
   An online textbook by Garth Kemerling, explaining the basic elements of elementary logic at an undergraduate level.
   http://www.philosophypages.com/lg/

Logic and Mathematics
   Essay on the nature of logic and its relationship to the philosophy of mathematics, written by Stephen G. Simpson.
   http://www.math.psu.edu/simpson/papers/philmath/

Nordic Journal of Philosophical Logic (NJPL)
   Edited at Dep. of Philosophy, Univ. of Oslo, published by Scandinavian U.P. in hard copy and on WWW. Full-text Web edition free for personal use.
   http://www.hf.uio.no/filosofi/njpl/

PHILOG
   The Danish Network for Philosophical Logic and its Applications.
   http://www.philog.ruc.dk/

Peirce's Logic
   Article in the Stanford Encyclopaedia of Philosophy, by Eric Hammer.
   http://plato.stanford.edu/entries/peirce-logic/

Symbolic Logic
   Online resources provided to supplement undergraduate course taught by Peter Suber. Good collection of handouts.
   http://www.earlham.edu/~peters/courses/log/loghome.htm

The Logic Course
   An interactive course in formal logic, with tutorials and quizzes on critical thinking, truth-tables, and sentential.
   http://www.thelogiccourse.com/

Tips on Translating English into First-Order Logic
   An introductory guide by Peter Suber.
   http://www.earlham.edu/~peters/courses/log/transtip.htm

Why Aristotelian Logic does not Work
   An essay at Abelard.org outlining an argument against Aristotelian syllogistic.
   http://www.abelard.org/category/category.htm