I use an app for Android called 'Moments of Gratitude'. I use the full version, it's only 99 cents:
If you have an iPhone, the 1000 Gifts app is the BEST one. BTW I highly highly highly recommend the book 1000 Gifts by Ann Voskamp:
If you are not a smartphone person 'The Secret Gratitude Book' is the best one I have found in a hard copy, pen and paper journal for gratitude. I use it as well when I am not able to use my phone.