By that definition, the killer apps for me are keyboard replacements, home screen replacements, flash enabled web browsers and the Android OS itself which allows me to modify/replace stock functionality. That freedom to do what >I< want to do with my device is the reason I went with Android.
I've tried several Firefox plugins, Outlook, and a few others that I can't remember and finally settled with Google Reader.
Not sure why you think it's horrible. It's always available from wherever I am, mobile or desktop. Adding feeds is as easy or easier than most other readers I tried, and it even creates feeds for sites that don't have them. Also, the sharing and commenting features are nice.