I don't know why this escaped me for so long, but Amazon has it's own site for selling comics in digital format and it's awesome. Comixology offers all the major comic series plus a lot of indie comics and graphic novels and the recommendation feature of the site is excellent. It has its own app for reading the comics that I like quite a lot and some publishers allow you to download DRM-free backups of your comics (for example Greg Rucka's Lazarus, a series I highly recommend). If you're in the US, you can purchase a flatrate that allows you to read as many comics as you like for a small sum every month, but that excludes DC and Marvel comics to my knowledge. Still, it looks like quite a nice deal.
I spent five minutes of Comixology and already had gathered a wishlist of at least twenty titles... It's great if you do not have a comic shop near you, if you just want to look into the first issue of a new series that you might be interested in oe if you just don't have the shelf space for printed comics.