For a comprehensive list of various materials that can show you how to prepare for the GED online, you should check out: http://research.umbc.edu/~ira/GEDres.html.
It has a great collection of resources for teachers (when taking a test, it's always useful to see how the teachers are thinking and how they will be rating you), and for students as well. The test links are divided in categories - for all the subjects, and just for the writing skills, social studies, science, literature and the arts, and math.
This is very helpful if you know which are your weak points, as it can help you focus just on those, and avoid the waste of time involved in preparing for the GED on subjects you already know. Each section has between 5 and 25 links, so you should find everything you need there.
For some good online test practice you should check out www.4tests.com which has good test samples of GED and many other tests, as well as nice collection of study materials and other useful links. It's always a good idea to take as many such practice tests as you can before the real thing.
Last but not least, if you're considering how to prepare for the GED online, or for any other test for all that matters, here's a site that can be of great help:
http://www.ucc.vt.edu/stdysk/stdyhlp.html provides great tips and tools for online studying. The site offers online study skills workshops and articles on topics such as time scheduling and management suggestions, concentration, control of study environment, how to take, edit and review lecture notes, motivation, and many more.
