I ported Quake 1 to the Apple Watch, building on top of existing ports for iOS and Mac.
Some features:
* uses Quake SW renderer + blitting to WatchKit surface (~60 fps, 640x480, larger res can run on lower framerate, tested up until 1024x768)
* touch + gyro + digital crown controls
* new AVFoundation audio backend (quake to Watchkit audio buffer copy logic), as Watchkit does not support CoreAudio
* high pass audio filter to remove clicking on Watch speaker for some of the low frequency quake .wav samples
* some smaller modifications and code updates to glue Quake 1 c code to Objective C and Watchkit
https://www.youtube.com/watch?v=cPC2o262TfQ
I once ported Quake 1 to the Rockbox operating system for MP3 players, which enabled it to run on the Apple iPod: https://www.youtube.com/watch?v=r6V-4AZ7pkA
The whole world of software ports basically boils down to "anything is possible, given enough memory, pixels, and time". The actual process of porting anything is sometimes incredibly tedious (e.g. "scour the entire source code for unaligned memory accesses, and manually rewrite all of them"), but also gives a chance to do some very clever things (like, "use the piezo speaker to beep out the address of a fault, because nothing else works on the device after a crash").
What made me do it was 1) the satisfaction I got from seeing my work produce something which, at first glance, shouldn't exist (Quake... on an MP3 player?), all while 2) learning deeply about software close to the bare metal, which is fairly hard to do early in one's career. There's of course also the factor of having lots of free time - I did my port when I was still a student with not very many responsibilities, so I could afford to spend entire evenings and weekends poring through mountains of C code. Nowadays, not so much...