We just found out about this deal, but the time is very limited, so act fast!
Archie seems to be laying it all on the line for the upcoming Worlds Unite crossover event, and that includes giving the first volume of the original Sonic/Mega Man crossover, Worlds Collide, away for free on Comixology! Just click here and enter the code "COLLIDE" (minus the quotation marks) to get the first four issues in collected format for free!
Mind that this deal is only supposed to be available for 24 hours, and the earliest we can find on it comes from Comixology's Twitter post marked 10 hours ago. So unless that was a late post, you've got approximately 14 hours left to take advantage of the deal.
Fortunately, all you have to do is enter the code, and the book will automatically be added to your Comixology book collection. Just as easy as that!
Update: Lanceheart directed me to a tweet from Comixology that reveals that the offer is actually still available, and will be until 11:59pm EST tonight (Saturday, March 14th), so you still have time to get in on this!