Most useful thing?
Honestly, a lamp that uses a 1W red LED behind a big diffuser. It uses PWM in the MHz range for dimming (so definitely no flicker), and big physical controls. My wife and I both get migraines and being able to set very dim red light seems to be better than sitting in complete darkness. I have insufficient data to tell if this is a real effect, unique to us, or placebo.
Code is in AVR assembly, because that's easiest for me. Sometimes I feel silly that after all these years working with technology, this is the most useful thing I've managed to build for myself. Hey, it's not nothing though :)