This website has online classes from MIT, Harvard, Berkeley, and many more. Most are short, interesting courses that can be audited or self-paced for free. Some include the option of college credit and/or more interaction with the instructors and college for a fee.
Hear and see fascinating educational presentations from universities, TEDx. science organizations and museums, hospitals, research institutions, Nova, the Science Channel and the Discovery Channel. On this site, all videos can be searched by keyword and have transcripts to read the spoken words. Although the transcripts were made by machine and have many unfortunate errors, they make each video searchable by keyword so that you can usually find exactly what you are looking for.
This website has free courses from the United Kingdom. They are designed to fit around your life, with short activities and clear goals to encourage you to make progress at a rate that suits you. It is easy to see how long each course will last and the needed time commitment.
Study a language online for free. This site offers Spanish, French, Chinese, English, Gaelic, and more. The courses are not the newest, but are still great (page is no longer updated but still active.)