At this time, there's no app for the 100-day envelope challenge. It would be very convenient to do this so you don't have to handle cash. But you could do it on your own. Here's how... You'd need 3x5 cards instead of physical envelopes, and write 1 - 100 on each one. You'll also want to set up a separate savings account for the challenge. Then every day, when you draw a card, you'd transfer that amount to your savings and then fill in your savings tracker. This would function the same; just how you hold the money is different.