I am not a huge app guy. My phone is relatively empty of them. There are two that I use daily.
Evernote: I am trying to rid myself of paper. I have stacks of notebooks that are training logs, work notes and journals. This blog became my journal and I have yet to give up pen and paper for my log but I am 80% paper free at work and a big part of this is Evernote. All my meeting notes and daily notes are put in here directly. I have notes for each retailer, the factories and employee. Business cards are a nightmare. I simply take a picture and it's filed in the correct place for future. Best of all is it's virtual so I have access to it on my phone, my work computer and this here Kindle.
Stress Doctor: I believe this is an I Phone app only. With stress doctor you are taking a daily resting heart rate and establishing a breathing pattern. The better the pattern, the more rewards or indicators that you are in low stress. It takes a few days to establish the patterns but once there it's gold. You wake up feeling great and rested, stress doctor will confirm it. You wake up feeling slow, stress doctor can confirm it. For an athlete you can use it to determine whether you need to back off or conversely pour it on. For the non-athlete (although Nike says "if you have a body, you are an athlete"), the stress doctor is probably better because as you know stress can kill. There are lots of apps for this purpose, I found this to be the easiest to use and understand.