While not every comic is suitable to be read in mobile form (full-page art is hard to appreciate on a sub-3″ screen), it is nice having a few comics around for subway or bus reading. I recently downloaded a huge pack of Four Color Comics, the original home of later hits like Dick Tracy and Donald Duck, and they are perfect for carrying around with you. Droid Comic Viewer reads .zip, .rar, .cbr and .cbz as well as plain image formats, so pretty much any digital comic backup will work. The app will go on any current Android platform as far as I can tell.
There are also a number of public domain or free-download comics available through Robot Comics as well, and they’re formatted for the reader already, so why not give it a shot?