Keep in mind that I am not affiliated with any of these, nor do I have any actual experience with them. I am just going to present some compiled information for you to consider if you are interested.
In my research, one site that consistently came up in the search results was Open Culture, who market themselves as "the best free cultural and educational media on the web." On their site, they have a list that contains 200 free online courses (which are "massive open online courses", or MOOCs) offered by various universities and institutions, some of which will issue a certificate of completion when you get through the course. Some instead will provide you a statement of accomplishment. These are good and worthy achievements, though they typically don't equate to transferable university credits. You can find things on this list such as Introduction to Astronomy, or Introduction to Finance... topics which are mathematical in nature. There are also classes offered with no certificates available, but may be more specific to mathematics, such as Introduction to Statistics. It appears that this list gets updated frequently, as the courses are listed monthly as they are available, but there are also courses available that have no fixed start date, and so you can sign up and begin whenever you want. I can't fairly comment about the quality of the material offered, but a brief look at some of the outlines available suggests that if you are serious about taking online math courses (or other subjects), these will teach you something.
Perhaps a more famous name than the previous site is the Massachusetts Institute of Technology (MIT), one of the more prestigious schools in America that you can attend. A few years ago, they announced the beginning of a new educational venture: MITx. While students that physically attend MIT (and gain the honour of being on campus, doing amazing things, and meeting brilliant professors), MITx promises to offer a similar quality of education online and for free. And even better, they have said that they would issue a certificate to show that you have mastered the subjects offered, though it appears that obtaining these credentials will cost a nominal fee. A quick glance at their site reveals a variety of topics available. Several other schools and sites have applauded MIT for this initiative, so hopefully other institutions will follow their lead and we will have more high-quality schools offering free, recognized education.
Some reports actually suggest that the concept of MOOCs is the beginning of something big in education. Another very well known school participating in this trend is Stanford University. They offer several courses to take online, with varying start dates and topics. A quick glance at the list of classes available online reveals several that are mathematical in nature, though most may be considered to be more advanced math concepts, such as algorithms and databases. Nonetheless, for a free course that will teach you something, you won't find a much more reputable name than Stanford University, so this list of courses is definitely worth keeping an eye on, especially as it appears to be updated periodically.
Beyond these famous educational institutions, there are a few more sites that I will mention here that are extremely popular with people looking for an online education. Coursera is a site that has partnered with top universities from around the world to offer free online courses to anyone who wishes to take them. High quality video lessons and interactive exercises, combined with a global community of like-minded students, help make this service incredibly attractive and useful. When I checked, there were 33 universities participating in the Coursera program, with a wide range of topics from music to medicine. Of course, mathematics is a dedicated section with many options available from which to choose. Algebra, calculus, and more advanced math concepts are available. Looking at the breadth of material available, it is easy to see why this service is so popular!
Similar to Coursera, Class Central is a MOOC aggregator. It appears to tap into the available course lists of many top universities, and makes them accessibly in an easily searchable database. You can quickly see if there are any online courses available for your selected topic with a simple search right from their home page. Class Central doesn't appear to offer any courses themselves, so anything you find there will be found offered by the originating institutions. I only include it here because it seems like it could be useful to you in your research on choosing an online course.
Finally, though it's the same as the online courses I've mentioned above, Khan Academy is a very popular and useful site that provides many video lessons on several topics across a wide variety of subjects. For example, in the math section, there are all sorts of videos that describe particular concepts, with very good explanations and examples. If you are looking for a quick lesson on a topic, and you aren't interested in going through a whole course to obtain credits or certificates, Khan Academy should be one of the first places you look.
Of course, if you are looking for help on math concepts, you can always search my site or contact me to see if you can find the assistance that you need. As I said, I have no experience with any of these services that I've discussed on this page. I have only done a basic research and compilation to bring this information together in one place for you to consider if you would like to further your education by taking free online courses.