TED website has been one of my favorites for several years. You'll find videos of many of the most popular TED talks there, all available for free viewing.
TED also offers video access through an iPad app I downloaded awhile back. I hadn't used it much till I discovered a very important feature: the ability to download and save TED talks locally on your iPad. I always figured I could access TED videos on their website as easily as through an app. There's also an option to download the talks from their website but the "Save Talk" button in the app is a much more convenient solution. I wound up downloading about a dozen talks initially, queuing them up at the same time and setting my iPad aside while they came in. I've only had time to watch a couple but it's nice knowing they're all with me and immediately viewable regardless of whether I'm online.
TED also recently announced another new extension you might be interested in. TEDBooks is a collection of short non-fiction works in digital format. The first three are being distributed via Amazon's new Kindle Singles program.