On my phone the only 3 beer-related apps I have installed are:
Jomebrew Beer Stats and Conversions
I grabbed this primarily to convert Brix to SG because my refractometer is in Brix. The interface isn't great, but its a quick simple app for doing some basic conversions. It will also take OG and SG and give you ABV and attenuation. It also does have a timer sections to it where you can set up a few timers for a step mash or boil additions.
I use their website for creating and storing recipes, so grabbed their app too. It is fairly new and has some bugs, but isn't bad for tweaking a recipe here or there, or if I'm out talking to other brewers and someone wants to know what I used in a beer I can look it up.
I think this app also has a brew-day section where it guides you through the recipe and each step, letting you track measurements along the way, but I never use it.
Which I mostly just use when tasting or researching, for style guide info.
I'm also a computer programmer and like my share of gadgets, but for me, I like just having a good old fashioned printout of my recipe and a spiral notebook to track my readings, measurements, and tasting notes for each batch.
The only thing I do on my phone on brew day is the brix->SG conversions and timers.