I've written before of Prof. Gordon Fee's online lectures on Galatians. Since then I've learned that Dr. Walter Kaiser, well-known evangelical Old Testament scholar [booklist here] and former President of Gordon-Conwell Theological Seminary, has a number of seminary chapel messages available online. They are all the more valuable for being Biblical studies of such books as Daniel, Deuteronomy, Isaiah, Nehemiah, Exodus, Matthew, and Hebrews. Other sermons of his are online also.