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.
Other
R. B. Jones: First Order Predicate Logic
Great introduction to basic ideas in predicate logic. Conveys fundamental ideas without using technical notation. A good place to start.
Stanford University
Temporal Logic/stanford Encyclopedia of Philosophy
Discusses the temporal logic of Arthur Prior, a type of modal logic. After technical definitions, author addresses related philosophical issues. Also summarizes applications in natural language, computer science, and artificial...