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

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/

Why Aristotelian Logic does not Work

An essay at Abelard.org outlining an argument against Aristotelian syllogistic. The argument focusses upon the relationship between Aristotle's categories and his conception of being, attacking them from an idealist position drawing upon Brouwerian intui
http://www.abelard.org/category/category.htm

The Troublesome Paradox

Online version of book seeking publication by Per Lundgren. Author attempts to argue that a consequence of Goedel's incompleteness theorem is that we should overturn our current approach to scientific method.
http://www.yesgoyes.com/

Logic

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

Peirce's Logic

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

Philosophical Logic

Introductory article by E. J. Loewe.
http://www.dur.ac.uk/~dfl0www/modules/logic/PHILLOG.HTM

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/

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

PHILOG

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

Tips on Translating English into First-Order Logic

An introductory guide by Peter Suber.
http://www.earlham.edu/~peters/courses/log/transtip.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/

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/