The GRE tests are well-known as a requirement for many graduate school programs, but for independent learners, the GRE subject tests serve the additional purpose as being an option for earning upper-level credit-by-exam. Offered in subjects like biology, mathematics, and computer science, these exams can give you up to 24 credit hours towards a degree in that major. The GRE subject test is one way of proving that you have the knowledge of a graduate in that respective field. There are numerous books and websites to help in preparing for these exams, including guides by the test's administrator, ETS.org. There's also an online simulator for all GRE exams available, which also offers the option of a free trial download. Of course, these preparation aids won't teach you the academic content of a degree in mathematics or computer science. Unless you already have a good knowledge of the subject, it's still necessary to study a good number of books on the subject. Any college website can give the information as to what topics the degree curriculum covers. Still, the GRE option is worth considering as a low-cost alternative to the traditional college route.