Image Comics are making over 100 1st issues across their titles available for free on Comixology. It looks like its for most series, and the majority of these have not been free before as far as I am aware.
For now they can easily added via the app by going to \Publishers\Image\Free\See All and adding them all.
Edit: Also available for Kindle (this link goes to a list of all the issues, and if your Amazon/Comixology accounts are linked, adding them for Kindle will also add them for Comixology)
Edit: And the excellent ComixologyFree Twitter feed now has the full list for Comixology.
I'll post a landing page for Comixology as soon as I find one. A few direct links to get you started: