Definitely andOTP, my two-factor authentication app for Android: https://github.com/andOTP/andOTP
The only thing cooler will be once I finished to rewrite it from scratch to get rid of the legacy code from before I forked it.

I have those conversations with my coworkers about once a day. We use Linux at work and I am the only one with any real Linux experience.

C: I have a problem! I tried and googled everything already! Come help me...
M: *slowly walks over to their PC*
M: *copy-pastes the error into Google*
M: *clicks the first result*
M: *presses two buttons*
*everything works again*
M: So you tried Google already, have you?

When I leave there (it's a PhD position and I'm almost done) they will probably crash and burn...

Best review I received for my app so far. It's a 2FA app, so basically every site where you enable 2FA tells you exactly what to do (scan the QR code)...

Rewrite my entire side-project from scratch using Kotlin and AndroidX. While doing so I want to learn about state of the art encryption and key derivation functions (especially Argon2).

Oh yeah, and finish my PhD...

Definitely the first Android app I decided to fork.
It was an open source OTP authenticator which hadn't been actively developed for 2 years at that point. At first I only did some small fixes and minor visual improvements but by now it's evolved into its own project with a lot of contributores and users on both Google Play and F-Droid.

When I started I had no knowledge of Java or Android development what so ever. So it basically forced me to learn lots of new stuff, especially once issues started to come in. By now I learned so much on this project that I'm thinking about re-writing the whole thing from scratch because I question some of the design choices from the original app I forked...

Github: https://github.com/andOTP/andOTP

AOSP

Even though Googles decisions around it are questionable most of the time, I love that we have an (almost) completely open ecosystem for smartphones.

And the thing about it I love the most is the amazing modding community around it.

I recently started learning Kotlin and while I like it a lot already I find it to be a really strange language at the same time. This is coming from someone mostly doing numeric stuff in Python (for my PhD) and Android development in Java (my personal side project, which I'm currently rewriting in Kotlin).