the one I use is Comix, but i think its linux only.
ComicBookLover is a fine product for mac, but it costs $25. My impression is you can still use it w/o buying, it will just show a big stamp on each page..
If you're coming from using CDisplay in Windows (as I was), the most similar app I've found is Simple Comic. Make sure to get the 1.7 beta, its perfectly stable and adds several new features.
Hmm...if you are talking about an online app to upload and read comics, this site might interest you www.mahshelf.com. Found this one recently through my buddies. The online reader's interface there is somewhat similar to CDisplay. But whereas CDisplay is offline, that one is online. Really neat.
I use Graphic Converter's slideshow for all my manga reading since I paid for it ($35) to have as a quick file editor already. Works great for viewing unzipped image folders and can handle thousands of images without crashing.