After being told about some free digital DC comics on the Google Play Book Store (mostly first issues to try and get you hooked, but a couple helpful looking graphic novel lists), I decided to check if they had the same deal on Comixology, and that's when I discovered a couple cool freebies (sorry this is probably US only).
Right now Comixology is offering a digital copy of the first two appearances and origin story of Wonder Woman!
First up is All Star Comis #8 (this digital copy only includes the Wonder Woman part of the original printing). This one is also available on the Google Play book store (but not the next part of the story).
After that is Sensation Comics #1, continuing the story of her arrival in man's world, with an injured Steve Trevor.
This is one of the best things about digital issues, being able to read the really old ones from the 1940s that are either non-existant or thousands of dollars for an original copy, or in a very expensive omnibus collection. Enjoy!