Einstein's Theory of Relativity needs no introduction. Actually maybe it does since we're not dealing with light concepts here.
Relativity in a Nutshell (MP3) offers a free, 30-minute introduction to Einstein's theoretical work. The lecture was presented by Richard Wolfson of Middlebury College. And it's coupled with a second free lecture called Einstein's Miraculous Year. Both lectures were produced by The Teaching Company to commemorate the 100th anniversary of Einstein's work on relativity (1905), and they tie into a larger commercial course taught by Wolfson: Einstein's Relativity and the Quantum Revolution: Modern Physics for Non-Scientists. The Teaching Company courses are not free, but they're well done. And if you buy the mp3 versions when they're on sale, you can often get the courses at a good price. Also, you can frequently find DVD versions discounted on Amazon (see, for example, here). And, even better, you can sometimes rent them for free from your local library.