There are two levels of participation, gold and silver. Most shops are going to be gold participants, but if you're lucky and have a large shop nearby you might also be able to snag the silver level books. These comics may be free to us, but they aren't free to the shops that are participating in FCBD, so don't bitch too much if your shop is a small one and all they have are the gold comics.
That Archaia Studios Mouse Guard book is actually really nice. It's not just a Mouse Guard story, it's an anthology with several stories from books the publish. It's also a hardbound book! For free! I don't even read any of their comics and I think that's awesome. I had to grab one just based on principle.
Free Comic Book Day is one of the coolest promotions still going on in print comics, and honestly I'm surprised it's still going strong, especially with the way that most companies are trying to push the industry toward digital comics. I'll be buying print for as long as they're printed though. Get out today and support your local comic shop! And maybe actually BUY a comic or two after you've picked out your free comics...