There are many free resources for online courses for whether you just want to brush up on a topic or learn a new topic completely. Some great sites for free online courses are:
KHAN ACADEMY: Khan Academy is a great resource for learning many new subjects such as math, science, and computer programming. It is organized in an almost game like fashion where you obtain badges and points depending on how well you are doing in the course. Courses start extremely simple and progressively increase in difficulty until you reach mastery level.
MITOPENCOURSEWARE: MITOPENCOURSEWARE offers free lecture notes, exams, and videos from MIT courses. It offers many in-depth courses specializing in Engineering and Science. This site is a great way to take courses offered by one of the most highly rated schools in the country.