The glaring, legitimate criticism of 3D printing technology is that, in its current state, it can only print "soft" parts, which largely limits it to things like figurines, mockups, models, etc. However, though the niche for useful 3D printed parts is tight, there are some genuinely useful builds.

Here's a collection of useful 3D printed stuff. Too useful? Then check out PancakeBot.

X-Acto knife multi-cap:

Bathtub drain removal tool:

Chapstick keyring cap:

A knob replacement for easier operation with arthritic hands:

Hose holder:

Tap case:

Washer fluid cap:

Folding door child lock:

Socket storage system:

Under-desk hidden holster. Don't play poker with this guy!

Drawer organizer for spools:

Multimeter probe caps:

Vise jaw cap: