If I were to choose one single currency trading app that has proven to be an excellent aid to trading, it would be the metatrader application. This is because of the range of useful tools made available to the trader. This includes the different indicators, expert advisors and graph presentations.
Of course the mileage varies depending on your brokers implementation. My MT4 is from OANDA and it has become practically the only tool I need aside from my news feeds and my mobile trading platform.
I mention the mobile platform because it is extremely indispensable to me. With the volatility in the market today, I never go anywhere without my netbook and one of my android devices which allows me to trade on the go.
MT4 is available on the Windows phone platform but for the past couple of years, I have been using Android exclusively, so what I use is the Android app of OANDA which I run on both my phone and my tablet.
There are other apps that aim to help the currency trader, but given the extremely fast paced market today, speed is of the essence. Hence I choose to keep things as simple as possible.