My left hand.

I discovered I can use it and do it well at Play Therapy training. We were doing an exercise as we were supposed to create something using the hand that we don't have a preference for in our habits (in my case, the left one). I do most things right-handed, so at first it was rather difficult to create using my left.
I chose to do a bracelet using colored paper and glitter glue, but without much confidence. I kept the right one behind my back and during the process it felt like it had a will of its own trying desperately to get into action. I tried not to focus on it, but to the left that had to handle stuff like cutting, folding, gluing, fitting. I also tried to follow my emotions as they grew from joy to anger... and then to joy and surprise again when seeing the result.
Everyone said the bracelet looked amazing and I think it did too. I somehow felt like a winner, happy and relieved after the achievement (although it doesn't look like much effort, it really is).
Ever since, I began using my non-preferred left hand in doing a lot of actions. It's really nice to discover they are both equally able, although one needs more practice.