Khan Academy. I've been hearing about this from members of my Plurk PLN for a while from time to time, but I had never looked into it until I saw the news story.
Sal Khan began by developing some videos to help his cousins with Algebra. He has gone on to develop over 2100 video lessons on a variety of subjects from Developmental Math to Calculus, Trigonometry, and Statistics.
These videos are now available to anyone, anywhere, any time, if they have an Internet-connected computer. They are posted on YouTube, and are easily downloadable for teachers to use as teaching resources, to assign for homework, or to even embed in your latest e-Pub book. (More on that topic soon!) Imagine creating a textbook for your students on each week's material, embedding the video examples in the text! It's so exciting!
On the Khan Academy site, you will also find at least 100 self-paced exercises. For this area, you must sign in, and you may elect to use your Google or Facebook account login. Once logged in, it will save your progress (your students' progress!) as you work through the exercises.
Sophia.org, developed by Don Smithmier. Here, you'll get an entire "Learning Packet" for your chosen topic, including videos, text examples, and links.
In the '60's and '70's "Free colleges" sprung up in many communities. One would simply walk in, sign up, and list what you could teach others, and list what you wanted to learn. Knowledge and skills were shared as a free exchange. These two websites remind me of that. People are actually sharing what they know freely with others.
If you know of other similar websites, please post a comment with the link. In this time of educational cutbacks, such resources are invaluable!