This should help you with loading apps that aren't in the Amazon appstore...
http://dearauthor.com/ebooks/yes-you...e-kindle-fire/
As for reading apps some of the most popular for ePub's are...
Mantano (supports Adobe & B&N DRM)
Aldiko (supports Adobe & B&N DRM)
BlueFire (supports Adobe & B&N DRM)
Moon+ Pro
Cool Reader
FBReader
Any of those apps should work just fine on your Fire.