http://comicstore.marvel.com/]Marvel digital comic store[url] - you can search for things you're interested in.
Marvel Unlimited - advertises over 13,000 comics, so hopefully some of them will be of interest.
orcishgamer: Damn, and digital comic books are such a damned ripoff too. I'd be happy to pay some cash for back catalog, but I'm not paying 10 bucks for digital "trades" with 3-6 issues. I forgot which, DC or Marvel, have slightly better deals (it's not fucking Dark Horse, that's for sure), but even those chafe a lot. I'd spend literally 100s of dollars to just go through one of the longer backstories since the beginning.
Marvel has Marvel Mondays, when they some dozens of issues (chosen stories) for $0.99 each. $1 and issue is also the regular price of the collections.
tritone: Does anyone know if the iPad app requires you to be always online to read each page?
I'm using the Android app, but no, you only have to be online to access the store, including seeing your comics in purchase order (which I find the most useful way to browse them, except in the case of these 700). Each comic is downloaded and you can read it when offline.
I've never been a big comic book reader, but I started reading them on the apps since I got my Nexus 7, and already bought a few first issues before this promotion. My plan is to find which ones I'm interested in continuing reading and then get a month of Marvel Unlimited to read more.
By the way, all the apps and all the stores (DC and Marvel) are powered by Comixology. You still have different accounts for each. I decided to get this promotion on the Marvel site/app to separate it from my Comixology account, even though I've already bought some Marvel stuff at Comixology directly.