Here is what I have loaded on my Kindle Fire:
IHeartRadio for listening to my favorite radio talk programs. You can start the IHeartRadio app and it plays in the background while I change to other apps like the browser for surfing the web.
Facebook app for keeping up with family and friends. It works just like my ipod touch and iphone. Actually I think it's just a browser link to m.facebook.com.
YouTube videos all day long.
Free public domain books from Amazon. Yes, Amazon sells free books.... I've purchased many, many books for $0.00 from Amazon. The checkout procedure is the same as any other book. The reciept shows $0.00. It's a great cheap way to find great books to read without having to cary them around.
Mint.com app is great for keeping me informed of how much money I have in the bank. The only thing I don't like about the Mint app for Kindle Fire is that I need to hit the refresh button. On my iphone and ipod the refresh is automatic when I launch the app.
Amazon app, ya sometimes I search Amazon on my Kindle Fire....
Ebay app. I do everything on Ebay using my Kindle Fire. I buy, sell, respond to request from potential buyers, setup watch items, bid.
Browse the web, endlessly.