Last year, I posted about Wolfram Alpha and the amazing computational document format developed by Wolfram Research. Recently, Wolfram Research has opened the Wolfram Education Portal, a site dedicated for storing and sharing dynamic teaching and learning materials. The portal contains free to use electronic textbooks in computational document format, lesson plans, widgets, and interactive Demonstrations. According to the website, a practice page with problem generators is currently being developed and will be online soon.
The website has several free sample teaching and learning materials but requires sign-up for full access.
The Wolfram Education Portal is currently in Beta, so I am not sure if the materials on the website will still be for free in the future.