Massachusetts Institute of Technology was a pioneer in offering online college courses and they still have the most diverse and in-depth collection of classes available anywhere. Their online offering is an archived version of real courses that have been taught at MIT, and you may need to purchase the textbook to follow along with the class. These courses come highly recommended.
Carnegie Mellon OpenLearning
Carnegie Mellon, based in Pittsburgh, PA, has a variety of courses available for anyone online. While the selection is not as great as MIT’s, many core courses are presented in an easy to follow online format. Their courses are similar to what a paying student would receive minus the course credit or access to an instructor.
Salman Khan began putting videos on YouTube to help with tutoring a cousin. When he started receiving feedback that they were helping a lot more people than just his cousin, he realized he was on to something good.
Today, there are over 1000 videos on KhanAcademy, which is now a 501(c)(3) non-profit. Most videos are about math. However, there are also many other topics that he explores. Even Bill Gates has said that he uses Khan to teach his kids about math. These are not college courses per-se, but the math he introduces in them are at college level and many people praise them for being clearer and easier to comprehend than actual college courses!
Stanford University iTunesU
Stanford has made many of their courses and lectures available via iTunes in iTunesU. This collection is great if you are on the go or want to listen to courses while commuting. They have an impressive collection of content available, all for free. The downside is that you need to use iTunes to view their content.
Open University LearningSpace
LearningSpace offers content from the Open University, based in the UK. This university pledges to be open and accepts many different kinds of students in its real world classrooms. It also opened up its content to anyone online. The online college courses are laid out extremely well and offer many different topics with skill levels going from beginner up to expert.