I've been using "
Learn and Master Guitar" - while I used to mess around on the guitar this course has taught me the fundamentals and really improved my playing. There is a lot of emphasis right from the start on learning to read music and understand music theory, which was all new to me. The course seems to be quite well put together and well-taught - not as good as having real lessons from a good teacher, but better than having real lessons from a poor teacher and easier to fit into a busy schedule. I haven't looked at justinguitar.com so I don't know how they compare - the Learn and Master course is not free for the basic course, but there are free "live lessons" on specific topics and a fairly active user forum. If you do go this route definitely get it directly from Learn and Master with the bonus lessons.