I was left with some nerve damage to my left hand after an accident a few years ago. I recently bought a Kinesis Advantage 2 ergonomic keyboard after. It's change my life. Haven't felt any pain or discomfort in my fingers/wrist in the past month that I've been using it...
A more comfortable ear pads for my headphones, especially when I am now 8+ hours a day in my headphones - they just become transparent for me.
Cheap, simple and yet it changed so much for me!