This is an eBook reader that can be installed on a PC to read files in the Nook format.  It is produced by Barnes & Noble. 

According to the web site, it can be used on PC's (Windows & Macs),
iPads, Androids, iPhone, BlackBerry.

I have only tested it on Windows XP Pro and on Ubuntu 10.10 using Wine 1.3.16.  It works with no difficulty on both of those platforms.