There are a lot of online, open course materials, e.g. MIT’s OpenCourseWare and Open.Michigan. OEDB.org’s Open Courses section is a bit of an aggregator and a good place to start any searches for online courses. Also, Open Textbooks specifically link to online, free, open textbooks.
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Hat tip to Daniel Strauss.
Lifehacker recently posted a nice compilation of open source education sites: http://lifehacker.com/5974371/plan-your-free-online-education-at-lifehacker-u-spring-semester-2013