is giving away a different issue for free every day between Dec 7 and Dec 18. (Link: http://blog.comixology.com/spotlight/
) Today's is Avengers Assemble #1.
They've also got an "all-ages comics" sale
which ends tonight, with 99¢-and-up issues of the likes of Archie, Bone, and presumably less-adult-themed supers. A few of these issues are free, including the first issue (not the first omnibus volume, which is $5) of Bone. If you have pre-reading-age kids, or just appreciate a good kids' book, take a look at Owly.
ComiXology has iOS, Android, and Kindle Fire-specific apps available.