Pick up a language, learn how to code at Codeacademy or just be inspired by the best Ted Talks. Apple's iTunes U is stocked with videos and podcasts of classes, as well as supporting materials like worksheets and ebooks. You can access them from a computer or download the ITunes U iPad or iPhone app. The YouTube Education channel has instructional videos on math, business, language and the other usual suspects, including fun experiments you can try at home. Check out the Spangler Science channel and prepare to simultaneously mess with and impress your kids.
Coursera offers free college courses from big name universities including Princeton, Emory, Stanford, Johns Hopkins and Columbia. Get the knowledge without the student loans (or course credit, unfortunately).