PassGED.com (see Resources 1) offers a comprehensive course that can be taken in the comfort of your home. While it does not guarantee that you will pass the test after taking the online class, it does offer a "pass or do not pay" option. In addition to the comprehensive class, you will also be assigned a GED tutor who will help you brush up on those areas in which you might not be so strong.
American Council of Education (ACE) offers several practice tests online to familiarize yourself with the kind of questions you'll need to be prepared for. You can click through the tests and find out why the answer you chose was correct or incorrect.
In addition to several books you can choose, Steck Vaughn offers a full range of GED practice tests on its website (See Resources 2). The fully interactive tests allow you to pick your subjects and take the tests you want to concentrate on. After you complete the practice tests, you will get a grade as well as an explanation of your answers. Although the books are not free, the practice tests are. You don't even need to provide an address or e-mail.
GED Online (see Resources 3) is another Internet-based course you can use for practice. Instead of a free test, this is an actual class. A four-month membership to the course is $65, which offers you not only preparation for the test, but also access to a community of other test takers. If your access to the Internet is unreliable or sporadic, you can order a CD-ROM to take you through the course.