I want to make it clear that I have zero involvement with these people, first. However, since we still do get an occasional "how do I get at the innards of my downloaded comic," and since it's always pretty scary to install new software, I think someone might appreciate a web service that has come to my attention:http://www.wobzip.org/
Yep. Web-based unzip/unrar.
It's not fast. It's limited to 100MB. It only recognizes the archive by extension (so rename "cbr" to "rar" and "cbz" to "zip"). But it does the job and lets you download the images individually.
(If you're particularly paranoid and dislike the RARs, I notice that, once you de-archive the file, you can "download all files as a .zip," thereby converting to a more familiar format.)
As I said, it's not exactly the greatest thing since sliced bread, but I figured that I should mention it for anybody who might need to use it.