Apple Editions can be read via the iBooks app on Apple’s signature line of iDevices.
Google Editions can be read on both Android and Apple devices via the Google Play Books app.
Kindle Editions can be read on a number of dedicated Amazon devices, like the phenomenal Paperwhite, and via free Kindle apps for Apple, Android, and Windows devices.
Kobo Editions can be read on a number of dedicated Kobo devices, like the Glo HD, and via free Kobo apps for Apple, Android, and Windows devices. [Additionally (and quite brilliantly), American bibliophiles can buy Hydroelectric titles through their favorite independent bookstores by creating their Kobo accounts here.]
NOOK Editions can be read on dedicated devices from Barnes & Noble, like the GlowLight Plus, and via free NOOK apps for Apple, Android, and Windows.