There's the butterworth mod but it's included in the Bunny version and only a minor mod of the MK3 stock one to correct the filament path.
Recently the PitStop extruder appeared as well:
What specifically are you looking to write on? General comparisons, specific features and capabilities?
Edit - paid mods, there is of course the BMG extruder, and the Bear extruder by itself, too, minus the gearbox from B&B
I recently installed the PitStop and I love it, you should definitely include it in your list!
After buying my MK3s I wasn't going to modify it as it works just fine for me the way it is. But ordered the parts for the pitstop extruder already and have most of the parts printed now. 😀
I'm afraid it isn't going to stay original very long. Maybe I need an other one or a mini as a backup. 😉
Well I felt the same way. I installed the PitStop just for fun as I was fascinated by the ability to simply pull apart the extruder. It now prints exactly as good as before. The build was easy. Enjoy!
I'm using Butterworth N4A right now. Sourcing parts to try a bear extruder soon. Nice thing about Butterworth is it's drop in replacement, no need to mod x axis as with bear.