Here are a few online audio lectures - with some background material - that you won't need to pay to hear:
1. ASTR 160 - Frontiers and Controversies in Astrophysics: Professor Charles Bailyn teaches this course in astrophysics that focuses on black holes, dark energy and extra-solar planets. [Open Yale]
2. PHYS 200 - Fundamentals of Physics: Those who have a good background in math and physics can get a great review from this course offered by Professor Ramamurti Shankar. [Open Yale]
4. String Theory, Black Holes, and the Laws of Nature: String theory provides promise in unraveling the mysteries that surround the laws that govern the universe and Professor Andrew Strominger discusses his insights into this theory and its relationship to black holes in this lecture. [Harvard @ Home]
7. A COMPLETE Search for New Suns: The COMPLETE project aims to map 1,000s of light years of star forming material in the Milky Way and you can learn all about it from Alyssa A. Goodman, Professor of Astronomy at Harvard in this lecture. [Harvard @ Home]
11. Observing the Birth of the Universe: Lyman Page, Professor of Physics, delivers this video lecture on the origins of our universe, using humorous and accessible means to explain complex concepts. [Princeton]
13. Einstein’s Biggest Blunder: A Cosmic Mystery Story: Alex Filippenko from the University of California, Berkeley delivers this lecture on one of the best-known thinkers and theorists of the 20th century. [Princeton]
54. Escher and the Droste Effect: Hendrik Lenstra, Professor of Mathematics, gives this lecture on the Droste effect, or the appearance of ever smaller pictures within a larger picture, giving a sense of infinite recession of space. [Princeton]
55. Matchsticks, Scramjets, and Black Holes: Numerical Simulation Faces Reality: Learn how the numbers translate to reality in this lecture from Elaine Oran, Senior Scientist for Reactive Flow Physics at the U.S. Naval Research Laboratory. [Princeton]