Some Available eReaders
Because they sell eBooks, it's not surprising that major retailers like Amazon and Barnes&Noble also sell the electronic hardware to read them. For example, Amazon sells the Kindle, B&N sells the Nook. The good news here is that if you don't want to buy an eReader, they provide an alternative: eReaders that can run on your PC or Mac. They're downloadable for free.
When you visit Amazon and select an eBook purchase, on the far right side of the page you'll see some links where you can download Kindle for Mac, Kindle for PC, and many others.
The Mac reader provides a comfortable viewing experience for any free or purchased books obtained through the Kindle Store. For me, however, I found it too confining: there was no way to open and read eBooks that I had already saved on my computer.
On other hand, the B&N Nook is much more flexible, and can read files from your disk. Download a free copy at Barnes & Noble NOOKbooks.
Once in awhile, it inexplicably crashes when I try opening some known-good epub files.
Preview (Apple Mac)
Some eBooks are delivered in PDF format, and I discovered that Preview does an excellent job handling the material. It's very flexible and can do far more than document viewing. If you double click on a PDF file (in Finder), chances are that Preview is the default viewer. The default can easily be changed in the Info window. You can also right click the PDF file name and select "Quick Look" in the pop-up menu.
Adobe Digital Editions
This free Adobe reader will display PDF and EPUB files. Download at Adobe. It has a somewhat simple user interface and provides a pleasant reading experience.
I find myself turning to this reader more regularly because it faithfully displays the correct colors and text formats found in the epub file.
If you have an epub file of a book you want to read, use the Firefox "File | Open File ..." menu and select your eBook file. Seems to work quite well. Get the extension by going to "Tools | Add-ons" menu, search for "epub" in the search box. By epubread.com.
Wikipedia has a huge collection of information on readers and editors. See their charts near the bottom of the EPUB page.