Modern States is a free site that offers free study materials to help you pass the CLEP exams; they do not provide credits directly. Each of their courses includes videos, readings/textbooks, and quizzes. By completing all of a given course, you are eligible to request a voucher that will allow you to take the CLEP exam for free but you must still pay the test center proctoring fee. Once you have finished the exam, you can submit your exam information and get the test center fee reimbursed.
In general, the materials provided by Modern States are extremely high-quality. Even if you are planning on getting credits from a different source, Modern States can be a great way to study for your exam(s). For instance, you might utilize Modern States to study calculus before signing up for Study.com to take their calculus course. Or you might use their Spanish course to prepare you for InstantCert's Spanish course.