Describe an app or program on your phone or computer that you find useful

The app I use most, and the one I'd struggle to replace, is a simple flashcard program that schedules reviews for you. You type in whatever you're trying to remember, and it decides when to show it to you again — sooner if you got it wrong, and at longer and longer intervals once you've got it right. I started using it for vocabulary in my second year and I've kept it running ever since. What makes it useful isn't the features, because it's actually rather ugly and has almost none. It's that it removes a decision. I don't have to work out what to revise; I open it, do about fifteen minutes, and close it. On a bad day that's the only studying I manage, and it's still enough to stop me forgetting. The drawback is that it's unforgiving — skip a week and you come back to four hundred cards waiting, which is demoralising. But I'd say it has done more for my English than any course I've paid for, largely because it's the only thing I've stuck with for three years.