Combining hobbies here, but I have printed a handful of genuinely useful things for my bikes and thought to list them out, none of these designs are mine but I have tested them and yet to break them
Ayup light handlebars mount. These are just a copy of the standard mount you buy
https://www.thingiverse.com/thing:1479926
Cable guide for Banshee V2 rear triangle, not exactly essential but tidies it up a bit
https://www.thingiverse.com/thing:2703451
Magura bleed blocks so much better than they things they give you in the bleed kit and you can adjust the thickness to whatever you want them to be.
https://www.thingiverse.com/thing:2854875
ZC44/EC44 headset press, used these as a one shot as I didn't want to buy and wait for a real press and knew I could do better than a couple of blocks of timber and a hammer
https://www.thingiverse.com/thing:3466884
Ayup light handlebars mount. These are just a copy of the standard mount you buy
https://www.thingiverse.com/thing:1479926
Cable guide for Banshee V2 rear triangle, not exactly essential but tidies it up a bit
https://www.thingiverse.com/thing:2703451
Magura bleed blocks so much better than they things they give you in the bleed kit and you can adjust the thickness to whatever you want them to be.
https://www.thingiverse.com/thing:2854875
ZC44/EC44 headset press, used these as a one shot as I didn't want to buy and wait for a real press and knew I could do better than a couple of blocks of timber and a hammer
https://www.thingiverse.com/thing:3466884