Marketplace
"For a limited time, Skybound is teaming up with Humble Bundle to offer the first three issues of The Walking Dead absolutely free. Start your comic collection off the right way, with one of today's hottest titles."
Not sure when it ends.
Pretty sure this was mentioned in the HB Walking Dead post here
Also for as little as 1c you can get the first volume of Walking Dead (issue 1-6) and three other 1st volumes of other comics