For the next couple days they will be giving away 700 different first issues of their various titles and series.You can find the free comics on the Marvel website, or at least you would if it had not crashed. They are also available via comiXology.
This is definitely one worth getting. The selection includes not just the first issues of long running series but also includes a number of standalone titles as well as the first issue of volume in a series. For example, there are free first issues from Book One and Book Two of Ender's Shadow (by Orson Scott Card).
The deal runs through tomorrow.