TidBITS and Peter Lewis have teamed up to fill an aching void in ebookland: an EPUB reader for the Mac—not the iPad, but the Mac. It's called Bookle, and it's really nice. You drag your DRM-free EPUB files to the Dock icon to load them into Bookle, and then start reading. Navigation is particularly fine with a trackpad, where you can swipe from chapter to chapter, and then scroll up and down for the contents of each.
I think what I love so much about the swiping is that it is almost like leafing through a print book. It feels right.
I can see Bookle being particularly helpful for proofreading ebooks before they're published since it's based on the same WebKit that iBooks is. The only significant differences I found between Bookle's representation of my books and iBooks' was that Bookle doesn't shrink images to fit the viewport, like iBooks does. That said, I haven't done enough tests to illustrate that point conclusively, but I have noticed it with covers, for example.
It doesn't do Fixed Layout (at least not properly), and it doesn't have Search, and you can't write notes or add Bookmarks, but given the fact that Apple may release their own desktop ereader app at any moment, I was impressed that they did all they could. You can vote for your preferred additions on Bookle UserVoice Forum.