If you're like me, it's easy to get carried away with your smart phone apps. I tend to try out a lot of technologies and applications; I'm a serial beta user and smart phone apps are no exception. That said, my "simplicity" DNA kicks in every week and I routinely purge those things that I don't use, need, or want.
Through all the technology noise, the most useful smart phone app for me is the Google Reader. I catalog what I want to read in the way that I want to read it...period. Any RSS feed reader will do the same trick, but I use the Google Reader (since I use it on my laptop anyway and my smart phone runs on Google's Android OS). The beauty of using a feed reader is that I can consolidate my favorite sites and blogs in one place, eliminating the need to go to each individual app.
There's nothing new to using a feed reader, they've been in our online world for a long time, so its extension into the mobile app world makes sense.