For the moment it's a Pi 4 + Hifiberry Digi+ Pro with Volumio software. Volumio provides a pretty decent web UI, automatic scanning of connected USB storage, Airplay (v1), and a Spotify Connect plugin. There are also other plugins but I haven't looked into them too closely.

I also liked Roon Bridge on the Pi (and felt the audio output was slightly improved, although made no effort to counter the placebo effect) but couldn't quite justify the cost.

If you have a Linux box like a Pi, you can expose that machine's audio device over the network with PulseAudio, and if you have that enabled, it'll autoconfigure and work like any other PulseAudio device.

I used to share my DAC between my laptop and my desktop this way. Whenever I plugged my headphones in, it'd default to them on the local device, and the remote would default to the network device. I could walk away from my desktop while playing something, and it'd continue playing on whichever I had my headphones plugged into.

I’m using a Raspberry Pi with shairport-sync, a SPDIF HAT and a Maestro-50 amplifier (that I got for cheap) and am really happy with it. I even route TV audio through it and have a IR sensor for remote control.