Enter this list, which has some great free samples from both the DC and Marvel comics apps, or a few other free Comics reader apps available on the store. There's some really good stuff out there for the low price of absolutely nothing, from some classic books that are hard to find in print to some newer promo books for current series or comics that have been made into movies.
Man cannot live on free alone -- if you're really into comics, you'll probably want to buy a few from their official apps (though it's a shame that pricing and selection isn't quite there yet, but hopefully Marvel and DC will eventually figure that out). And this article doesn't even mention the tons of great single-app books you can find on the App Store, for both the iPhone and the iPad. The iPad was designed for consuming media, and comic books are one of my favorite media around.