What App Is Your Favourite Hidden Gem?

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Chances are you have hundreds of apps installed across your smartphone, tablet and computer. And many of those will be common apps that lots of people use. But there's probably a small subset of apps that you;ve found that not many other people know about. So, I'm asking what's your favourite app that very few people know about?

A recent post at Product Hunt asked this question and reveal led some really handy utilities.

I've been using a computer for study and work for over 30 years and I've been fascinated with how a small utility or quirky application can make a big difference to my productivity.

Of the ones on the Product Hunt list, I've used Grammarly before and will give f.lux a try. But what apps are you using that aren't mainstream but you find super useful? Let's share them, noting what platform you're using them on.