There will be a lot of overlap in what each finds, but it may be worth trying more than one when you aren't sure what you are looking for.
When you find a course that looks interesting, be sure to go out to the actual website offering the course for full information, such as whether or not it is currently available for enrollment.
Note that some search engines will display courses that cost money; there is almost always a way to "filter those out" of your results so that you can only see the free courses.
CLICK HERE for an IMPORTANT NOTE about Course Search Engines. Free online courses/MOOC aggregator - Class Central. CourseBuffet | Find and compare free online courses from Coursera, Udacity, edX and more.
CourseTalk | Search For Online Courses and MOOCs ratings and reviews. RedHoop | Online Education, Courses, Search, and Reviews | Knollop. OCW Consortium | Courses. My Education Path :: Free Education Planner, MOOC aggregator, courses reviews and comments!