Letter to Allah is a free and offline app that lets you write your heart out to Allah in the form of letters. No need to sign up or even have an internet connection for using the app.
Like our prayers, actually. This is a great idea, I care about it. If we write down the promises we made to our Lord in a letter, we can remember the promises we forgot or did not fulfill in the future. No one wants to be ashamed of Allah, right?