1. Media player connected to projector running RasPlex - this software is outdated enough and buffers on some high bitrate content that I should buy a replacement device, but it still works well enough. I tried upgrading to a newer raspi and wasted an hour trying to get it to run, then gave up. So, I still use my old one. It still gets used daily and works well enough (only issue is the buffering on occasion).
2. RetroPie - I rarely game, but it's cool to be able to turn this on and have a library of all the games I played (and those I never had) from childhood.
3. I use the third one as a networked LED marquee controller (HUB75 panels) with this software: https://github.com/hzeller/rpi-rgb-led-matrix
The other 2 I just have sitting on my desk and occasionally use for small dev projects or to test out some new project I read about on here, hackaday, etc.
An ongoing project that I haven't made much progress with is an automated turret that squirts squirrels with water. I made something similar in college (a "paintball gun" turret with openCV blob detection/tracking) that had decent performance. Now that openCV on rpi can outperform my old college laptop, I want to setup the pi to detect squirrels, track them, and keep them away from a bird feeder/plants in my backyard.