Perfect Viewer is a free app that allows one to view comics in various formats, including CBRs and CBZs (renamed RAR and ZIP files containing JPGs in sequential order), as well as PDFs and JPGs. There’s support as well for Japanese manga in its original format, with an option for right-to-left reading. A virtual bookcase (akin to the one for iBooks on iOS) is also offered, displaying the covers of various comics.
One flaw is that the controls are slightly awkward/hidden, making use of various hot spots around the screen.
While reading on a smartphone isn’t the most optimal screen size, Perfect Viewer does scale up nicely on an Android tablet. I made use of Perfect Viewer on my formerly-rooted Nook Color, which I used to read comics while on trips.
Perfect Viewer, of course, doesn’t support purchases made on digital comics stores like Comixology, which has its own proprietary (read: digital rights management-laden) web site viewer / smartphone app to read its comics through, though the Comixology app is otherwise nicely done.