The Khan Academy is an online non-profit organization whose goal is to provide a "free world-class education to anyone anywhere." Think of it as TED for everyone, except Khan's videos, resources, and lesson plans can actually help you be one of the TED speakers one day. Currently the Khan Academy is only accessible through a browser, but Mozilla's John Resig has shown off a few alpha screenshots of the upcoming Khan Academy iPad app, and it looks awesome.
Initially the 1.0 version of the iPad app will allow video navigation and viewing, interactive transcripts, and offline support. However, future versions of the iPad app will allow for in-app exercises
