University of St. Andrews (UK)
University of St. Andrews: Friedrich Ludwig Gottlob Frege
Great summary of Frege's philosophical significance in terms that anyone can understand. Includes links to his works and several illustrative quotes. A great place to start. Includes links to related philosophers and logical topics.
Other
R. B. Jones: The Philosophy of Logic
Excellent overview of logic. Defines the term in informative detail, while relating logic to its neighbors in epistemology, metaphysics, and analytic philosophy. A terrific introduction to the subject.
Stanford University
Stanford Encyclopedia of Phil.: Principia Mathematica
This site from the Stanford Encyclopedia of Philosophy briefly explains the history and significance of the famous logical treatise by Whitehead and Russell. Lists the table of contents from each of its three volumes.
Curated OER
Philosophical Logicism
Excellent overview of logic. Defines the term in informative detail, while relating logic to its neighbors in epistemology, metaphysics, and analytic philosophy. A terrific introduction to the subject.