Left-handed, but after years of not being able to mouse with the left hand, I can only do that with my right. Still, considering that I use a trackpad a lot, I do that left-handedly.
You wouldn't think about it, but even when the port for a mouse is not automatically on the right side, computer desks are almost always designed with the towers on the right, meaning that if you even try to use your left hand, you run out of cord. Not to mention that "ergonomic" mice hurt pretty quickly in the wrong hand.
Also, while I can't write with my right hand, I find that I sometimes start randomly painting with my right hand, though this usually happens when I'm painting a lot of straight lines. And I can type one-handedly with both hands.