Stanford University
Stanford Encyclopedia of Philosophy: Gottlob Frege
Good summary of Frege's thought. Some sections are more technical, but the explanation of his philosophy of language and other sections are more readable.
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.
Stanford University
Stanford Encyclopedia of Philosophy: Frege's Logic
Lengthy and detailed, but also quite technical, article explaining the major theorems of Frege's logic. Introduction gives a helpful overview and bibliography is included at end, but anyone without a background in logical notation may...