Groove Mount Bowden Adaptor (1.75 mm) - E3D
This is a neat little solution for quickly and easily converting a direct 1.75 mm filament extruder that would normally accept a groovemount hotend into a bowden extruder. Simply mounts in like a hotend, but instead accepts 4mm PTFE tubing securely in an embedded bowden coupler.
Quite a bit nicer than forcibly screwing a threaded bowden coupler into a printed hole, and allows you to use the existing plethora of excellent direct extruders available in a bowden setup.
If you're designing your own bowden extruder you might consider using this to secure your tubing instead of a screw in coupler.