There are a number of games I really enjoy more on the Fire...
Majesty: The Fantasy Kingdom simulator (which is now available properly) I really like except it sometimes just quits on me, annoying so I save every few minutes. But it really works well on the fire screen.
Even though I was annoyed at paying for it again as a separate version I really like Plants vs Zombies as well.
FieldRunners isn't my favorite tower defense, but it does look incredible on the Fire.
Non-game apps I use and really enjoy:
AndChat (sideloaded). There's an IRC chat I like to hang out in while watching football (football outsiders), and andchat lets me do this very well, no need for the laptop now while watching in my living room.
Go Launcher - I still primarily use the kindle launcher, but Go is great for those times when I want it more android-like.
Nook Reader - I use this for epub documents, of which I had a number. Installed via getjar.