This question already has an answer here:
Adobe Digital Editions has some capabilities but is essentially an orphaned product with no support.
Stanza only displays text in ePub books, and again, is an orphaned product with no support.
Calibre is mostly for building e-books. It has a text-only ePub reader but it's even more primitive than Stanza for Mac.
Scrivener can compile and output ePub format, but I don't think it can display them for reading on your Mac.
There are various apps for iPad and iPhone, but I already know about them.
And let's not discuss DRM issues; what we all want is a reader that will let us display ePub books (without DRM, or perhaps certain forms of DRM), one that supports displaying significant formatting and graphics and photographs if present.
Is there anything out there at present other than what I've noted above?
Let's exclude Window, Linux, or anything that would require X11. I'm hoping somebody knows of a native Mac app, or something you can run in a Web browser.