Classical Logic - Introduction to classical logic, including completeness and Löwenheim-Skolem theorems; by Stewart Shapiro.
Computability Logic Homepage - Main online source on computability logic - a recently emerged formal theory of computational tasks and resources. Maintained by G.Japaridze.
Logic - Open Encyclopedia entry. Hierarchically organized by subareas.
Math Front: LO Logic - Front end for the logic section of the mathematics e-print arXiv.
Mathematical Logic and Foundations - From The Mathematical Atlas, a resource of mathematics maintained by David Rusin. Extensive resources related to logic and set theory.
