Hi, I think this is it..
Since you specified ePub, we will ignore Amazon. All of these answers apply to DRM'd versions. non-DRM should work on any device that supports ePub.
1. Apple ePub: Ipod Touch, Iphone and IPad.
2. Barnes and Noble ePub: Nook, Jetbook, Jetbook-Lite and I believe the Pandigital line of eReaders as well as on B&N software for Android, iOS, Blackberry, etc.
3. Adobe Digital Editions ePub: Pretty much all other readers plus the readers that support Barnes and Noble ePub.
If we removed Apple from the equation, the relationship between B&N and Adobe would be kind of like the relationship between AB blood and O blood. B&N enabled readers can read the most DRM'd formats, while Adobe eBooks can be read on the most readers
Hope that helps.
Oh Right, one last detail I forgot. Pretty much every store outside of Apple, Amazon and B&N supports Adobe Digital Editions if they sell DRM'd books at all (Not counting obvious exceptions like stores specializing in mobi or even .lit.