timely article, I have plans to build a free standing lean to in the spring, something like a 20 x 20. My idea is to have a second slope at a steeper angle to allow an overhang at the open end to help keep weather out. Basically if you can think of it, someone has done something similar and put it on the internet. Try a google search, read how to books from the library, etc.
Bill's buildiing shows sturdy construction, but could be scaled down to yard barn size if you want