Modern States offers free online college level courses in multiple disciplines.
Sign up. Take the course. They issue you a free voucher to take a Clep test.
Check the Clep site to learn more about the tests.
Here you can see what tests are accepted at schools you are interested in.
NOT Free, but low cost option:
If you are a high school students with a 3.0 GPA or higher and at least a 24 on the ACT, you can apply for low cost SELU classes http://www.southeastern.edu/apply/high_school/index.html.