Too many to list, but special mention for the “PCalc” calculator for my iPhone. Many keyboard options to choose from: simple, scientific (algebraic or RPN), on and on… And it’s great for unit conversions. With most keyboard selections, turning the phone to landscape orientation adds more function keys.

I generally have the RPN scientific keyboard up.

Edit: And the Amazon app. Wonderful, wonderful!

And the mSecure password vault. This, or some other PW vault, is absolutely essential. Syncs with the same app on my Mac Book.

Storm or Storm Radar

ISS HD Live - Feed from the International Space Station

WIFI Analyzer

Phyphox - Let's you use see/use all the individual sensors on your phone. Kinda neat to just mess with.

Gunbroker

Gun Vault Pro - Inventory your firearms. Including serial numbers, photos, number of rounds through it, etc...

My must-have, most-used apps:

Chrome
Google Maps
Wunderground (weather)
gTasks (to-do list)
Audible (audio books)
Touch 11i (HP-11C emulator!!!!!!!!!)
Yahoo Finance
eBay
Cribbage Pro
LinkedIn (professional networking)
Tapatalk (kinda sucks, but the best option for forums other than SigForum)
Netflix
Scanner Pro (document scanner to pdf)
Fox News
Wunderground (weather)
DropBox (cloud storage)
LastPass (password manager)
Tidal (subscription music)
Starbucks (order ahead, no waiting!)
Emerald Sequoia (atomic clock server for setting watches)
Various financials (banks, credit cards, retirement accts etc)

Overdrive and Axis360: both are apps to search for, and borrow, audiobooks, and e-books, from libraries.

Previously I exclusively used Audible for audiobooks. I was using two credits a month, and a few daily deals, which meant I was spending $275+ each year. Now with Overdrive and Axis360, I'm getting most of my books free. I still get one Audible credit every other month (6 per year), which only costs $90 each year. I do that for the rare occasions the local libraries I belong to don't have a title I want.

It's a huge savings, and I listen to books most times that I'm alone in a car.

This message has been edited. Last edited by: DMF,

Besides apps that may come pre-installed:

Soundhound and/or Shazzam
TuneIn Radio
Netflix
GPS Status (may be Android specific)
Evernote
Waze
Various bank/shopping apps specific to my needs

Well, my needs are simple so my most used list:

KeePass Touch - Secure password manager. Synchronizes through Dropbox which keeps phones and computers up to date
Dropbox - see above and to share big files
GassBuddy - best way to find low price gas around you.
Waze - best nav tool around
VNC viewer - to control my various Pc's and macs
Tile - finds my lost keys, parked car
Sensi - controls air/heat in my 3 houses
WiFi Sweetspots - measures WiFi transfer speeds
Yahoo Stocks - to see how much money I lost
ProCamera
TuneIn Radio