As you might expect, the free digital editions included famous titles such as The Avengers, X-Men, and Iron Man, as well as less well-known titles such as Guardians of the Galaxy, which should become better known once the movie version releases on Aug. 1, 2014.
The promotion is being tied into the company's appearance at the South by Southwest Conference (SXSW), and it thus only runs for a short time. It kicked off on Sunday and runs through 11 p.m. EDT on Tuesday. The titles can be "purchased" through either Marvel's official comics app, its web-based digital comics shop, or through its partner, comiXology (which also has apps, as you would expect).
The titles go from 1960s Silver Age versions to contemporary issues. That means, for example, if you wanted to, you could read the original versions, rebooted versions (and there have been many) and the the current Marvel Now titles.
However, the demand is such that both stores are experiencing issues loading. You've got until Tuesday night at 11 p.m., but that might not seem that long if you keep getting site errors.