The College-Level Examination Program or CLEP is an exam set up to give United States college students the chance to completely circumvent certain classes and gain full credit for them by taking a relatively short (about 90 minutes) proficiency exam.
The CLEP test is offered in most community colleges and universities. You can take one anywhere from Valencia Community College (Orlando, FL) to University of Massachusetts (Boston, MA). There are many different CLEP subjects to choose from, though they are all more or less entry-level classes. So even if you know quantum physics the highest one you could take would be Natural Sciences. The amount of credits you receive also depends on the score and the college where you took the test.
The five major categories are Composition & Literature, Foreign Languages, History & Social Sciences, Science & Mathematics and Business. Each one has subcategories labeled after actual courses, i.e. U.S. Government, Freshman Composition, etc.
The $50 (plus registration fee, approx. $15) cost of taking a CLEP exam is very reasonable considering you would pay multiples of that amount for a regular college class, especially if you are paying out-of-state tuition. Members of the U.S. military are even allowed to take the test for free.
In essence, the CLEP is a great way to save a lot of time and money by getting a lot of the basic courses out of the way. Even if you don't have any or enough knowledge of the subject, studying for 2-3 weeks and taking a quick exam beats sitting in class for several months and paying at least twice as much.
Several CLEP study guides are available through the usual sources (Amazon, etc.) and coupled with a textbook you stand good chance of passing.