Here's a few great resources, including links to some lectures I've enjoyed.
Listening to Words - fantastic clearinghouse for all kinds of different lectures, gathered from various sources across the internet, including MIT, Princeton, the Museum of Natural History, and the BBC. Plus you can browse by lectures, people, locations, tags, and popular.
- Bill Bryson - A Short History of Nearly Everything [mp3]
- Jared Diamond - How Societies Fail-And Sometimes Succeed [mp3]
- Jill Tarter - The Search for Extra-terrestrial Intelligence: Necessarily a Long-term Strategy [mp3]
- Richard Dawkins - The universe is queerer than we can suppose [mp3 | mp4 video]
- James Nachtwey - The recent TED winner discusses his photography [mp3 | mp4 video]
You can practically attend the world's best university online for free.