Said it before, sayin' it again...

[The Rat]

Never throw anything out! Okay, maybe not anything, but bits that look useless can sometimes come in handy. Example: in order to reduce the chance of a short-shot, plastic is often squeezed into a tiny round recess past the part. I have a cache of these, and one piece came in handy when I needed to scratch a teeny headrest for my Revell Boeing P-26. Looks fine to me!

[kerick]

They make nice little greeblies on all sorts of projects. I have a bunch saved up.

[zenrat]

I have hundreds in the stash.  Still attached to parts of unbuilt kits...

<smart donkey mode off>

[Wardukw]

Same here ,,i keep sprue parts from all sorts of frames ,,,ya just never know now do ya ,,,like the sprues from Warhammer 40K kits ,,there Trapezium shaped and yes i did have to look that up 
But also half rounds ,,smooth rounded corners of different thicknesses .mold release pins come in all shapes and sizes ..again ,,very handy .

[Old Wombat]

I've also, started keeping the bases from some 3D printed parts, along with lots of the support structures.

The bases, depending on their shape, can be used for anything from flooring, to hatches, to bases for figures.

The support structures can be used for all sorts of things, from pipes to .50cal cocking handles (Don't ask! ), to support structures ( ).

[Rick Lowe]

I've also, started keeping the bases from some 3D printed parts, along with lots of the support structures.

The bases, depending on their shape, can be used for anything from flooring, to hatches, to bases for figures.

The support structures can be used for all sorts of things, from pipes to .50cal cocking handles (Don't ask! ), to support structures ( ).

The older types can be used as Grates for sci-fi/industrial flooring, or pipe ends; lots of uses.
Of course, often they droop between the frame lines, and you need to have the knack to separate them from the parts without destroying them, but it's doable.

I think the newer organic supports could be cobbled together into sci-fi trees; I keep meaning to have an experiment.

And yes, I've used the little nubbins for headrests, wheel hubs, etc; they have lots of uses.

[frank2056]

I think the newer organic supports could be cobbled together into sci-fi trees; I keep meaning to have an experiment.

I have a bag with the organic tree supports from my filament printer. They do look like trees and they reduce the amount of material needed to support parts. Here's what they look like (not my print):

[Rick Lowe]

That's them; a coat of white glue and sand, and you'll not see the lines.
And it will hide the joins if you attach more branches. 

[NARSES2]

Some of the larger ones look as though they could be used for some weird alien's arms & legs, lower torsos etc.