Comixology is having a special sale today in honor of National Comic Book Day. They're giving away 26 issues of various series, including Unity, Shadowman, Harbinger, and more. In order to get the free comics, you'll need to first add them to your cart and them enter the discount code Comics before you check out. The code is only good until midnight, eastern.
Continuing my quest to never pay for the comics I buy from Comixology, I got all 26 titles, and I can confirm that 18 are also available as DRM-free downloads. This means that you can protect your purchases from the unexpected.