The Wikipedia app. I swear I've learnt so much more by thinking "Oh I wonder how X works?" "What does X mean?"
If there's something I realise I don't know, I just grab my phone and find out. I'll go to quickly check something, usually something biological like a disease or a sometimes an organisation, and I'll be there for hours, clicking through links to learn more and more.
Wikipedia is the best. I know anyone can edit it and pollute it with misinformation, but it's so well managed these days that it's just not as unreliable as it's made out. Anyway, I use it all the time and I'd be lost without it.
Computer-wise, I can't seem to be without Grammarly. It's a little grammar and spelling checker that sits in the background, sifting through everything you type everywhere, making little tweaks and ensuring you're using the correct terms. It's incredibly useful in school and in places like forums as I don't even have to intervene most of the time, whenever I finish a sentence, the sentence fixes itself if there are any issues.
A close second would be Kodi. As someone who regularly sifts through my YT sub box, listens to music for hours, and binge-watches anime, having everything accessible from one, clean and optimised interface is ideal. I have a little 2GB sd card that I just stick into my laptop (or anyone else's laptop) and I have an entire media centre with everything I need, all elegantly laid out. So much handier than using lots of different programs that all form a crude system with far too much going on in the background. And because I use an old laptop, it needs to use all of its strength without background distractions to deliver smooth even just 720p video