The following downloadable applications are free:
Kindle Reading Apps (cloud, desktops, smartphones & tablets)
Kobo e-Reading Apps (Android, Blackberry, iPad & iPhone, Instant Reader, Mac & PC)
Nook Reading Apps (Android, iPad & iPhone, Mac & PC)
Some publishers and online bookstores also offer e-books in "PDF" format, which can be opened using the freely available Adobe Acrobat Reader.