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.