ADA Project is a great resource being developed by a mathematics teacher named Sam Powell. The ADA Project is an open multimedia mathematics textbook that covers everything from basic arithmetic through calculus.
When you visit the ADA Project's homescreen you can choose a category then choose a topic. Within each topic you will find a set of sample problems. Each sample problem is accompanied by a link to reveal the answer, the solution, a video about the solution, and a link to a discussion forum. Take a look at this set of long division problems to get a sense of how the ADA Project works.
Teachers are invited to contribute to the ADA Project's development by submitting problems, solutions, videos, and discussions. You can submit one or all four of those pieces for inclusion in the ADA Project. The submission form is found here.
Applications for Education
Although it is off to a great start, the ADA Project is still a work in progress. At this point it will make a good supplement to the textbook and other reference materials that you use in your mathematics lessons.
The ADA Project will get better through the contributions of other mathematics teachers who make submissions to it. Contact Sam and make a submission to help the project along.