I'm sure lots of us have made useful gadgets for particular jobs. Why not enter your ideas as answers to this post?

Very small set-square, for awkward corners.
Take the outer case from an audio cassette, and saw off an appropriate angle, to any size and width you wish. The clear plastic part is extra handy.
Just be careful to use a sharp saw, and keep it straight - the plastic snaps and cracks very easily.

Looking forward to your replies
Modulla

Toothpicks
have many uses. When painting wheels, put a tiny piece of blue tac on the tip of the pick, and the wheel on the Blutac. Paint the wheel. By rolling the pick in your fingers you can spin the wheel while holding the brush steady and get a smooth circle. Now put a larger blob of Blutac on your workbench, and the free end of the pick in that, so the pick is vertical, and leave to dry.
If like me you sometimes drop things on the floor, paint alternate colour bands (eg red/white) on the toothpick so it shows up on the carpet.

Scooty bag
When cutting things to size, pieces often fly off to the far horizon. Do your cutting and snipping inside a clear plastc bag.

Hi Clive

Not so much a tool, as a free source of material. We get through a lot of DVDs at work and the stacks come with blank "spacer" disks top and bottom. The material used varies, but some have very good quality clear styrene disks.

Al the best

Rowan