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.
http://www.abelard.org/category/category.htm

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/

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/

Dead Logicians Society
Warwick university's very own logic society aiming to increase and stimulate interest in logic and philosophy of logic at the university. Offers news and events.
http://www.freewebs.com/deadlogicianssociety/