Each day, ComiXology will be giving away a free digital comic. The comics go live at Noon EST, at which time the previous day's comic goes back to its regular price.
They started off with a bang this year, giving away Secret Wars #1 (a $5 book). You still have time to grab it if you get over to www.comixology.com before noon. Check in each day and grab your free book, as there are bound to be at least a few series you probably haven't read yet.