Or, if you're too cheap to pay a buck or two for a full app, you can even do it yourself. .cbr files are really just .jpg archives, so as this tip from a while back on the iPhone Comic Book Reader says, you can actually extract out the jpegs, and then just sync them up to your iPhone via iPhoto if you want. Three dollars really isn't that much, and any of those apps listed above will give you a little more functionality (easy flipping between pages, bookmarking, and many other features per app), but if you really want to DIY, there you go.
Now it would be nice to be able to officially buy .cbr archives over the web from some of the major companies -- the closest they've come that I know of is a subscription service, but of course you've got to use their own client for that (and there's no iPhone app that I can find, though Uclick has a few apps in there for certain books). As long as reading pirated .cbr files is outlawed, only outlaws will end up reading them.