To open them on an iPad, iPhone, or iPod touch, you just need to make sure you have the newest version of iBooks installed.. then click on the “Download eBook” link for the book you want!
To open them on a computer (all platforms), we recommend first using the Mozilla Firefox Web Browser, then adding the free EPUBReader plugin.
You could also use the free Adobe Digital Editions software (use CTRL-O to add ePubs to the in-app library) to read them on a computer (Windows and Mac OS), but it’s a little weird sometimes.
If you have a Barnes & Noble NOOKcolor download the epub using the built-in web browser, then go to “Library > My Files > My Documents” and tap the epub files there! (You can also copy them via USB like you do with the original Nook.)
On an original Nook or Nook Simple Touch download them to your computer, then connect the nook via USB and copy the epub files to its “Documents” folder!