Comixology giving away a free digital book a day as they approach 100 million issues sold. Up first: Spider-Men #1
Free comic book per day, starts with Spider-Men Issue #1
The site appears to be American but it worked fine for me, just make an account (takes about 20 seconds) and add the code "#CountdownDay1" (minus the quotation marks).
Not entirely sure how long each free comic is available for, could be today only, could be longer.