Android: Android's designed largely for mobile users, but since it has Linux at its core, it can work with a desktop environment as well. If you've ever wanted a Linux-style GNU workspace for Android, this guide from XDA can show you how.
The guide at the link below walks you through all the steps to install GNU on any Android device. While you probably don't want to do this on the phone you use every day, you can repurpose old devices to get a lot of use out of them. If you want to turn an old phone into a low-powered media server or transform a tablet into a fully-fledged laptop, a proper Linux install can help.
At the end of the guide, you'll have a Debian Linux environment running within the Android application sandbox. The upshot of this is that you don't need root and you can install it without wiping your phone. If you ever want to go back to regular Android, it's still there. You won't be able to use every single app available on Linux, as Android's custom version doesn't make some under-the-hood features available that many apps need. However, as XDA points out, you can still use apps like Firefox, LibreOffice and GIMP. Check out the full guide for instructions below.