For the most part, I'm left handed. I say for the most part, because there are certain things you learn to do with your right hand when you grow up left handed (or at least if you grew up 20+ years ago). I learned to throw a frisbee right handed. I bat with both hands. I can write on a chalkboard better with my right hand. And I learned to use a mouse with my right hand like every other kid.
These days, I use the mouse with my left hand. It wasn't out of choice, though. A few years ago my arm started to hurt like hell when I touched a mouse or typed. I guess it was from many years of using computers (a lot of which were Counter-Strike in college). At one point about three years ago it got so bad that I couldn't do work at all. My arm, my shoulder, everything hurt.
I went to the doctor and they told me I had damaged the tendon and bone on the inside of my elbow from repetitive motion. It's called medial epicondylitis, or, to get all layman on you: Golfer's Elbow. Which is actually really annoying because I don't play golf. Basically, I was kind of screwed. The primary thing doctors suggest for this is rest and icing and taking some time away from activities. But I can't really do that.
So I switched to using my left hand. I know how that sounds—I chose to begin injuring my left hand instead. And that's sort of true. I figured I had at least a few years of use before I get into the same situation on that side. That, and I have made a conscious effort to be smarter about how I use the mouse, my computer, my seating, everything. I've taken much better care of my left arm in the past two years, and I've experienced no pain thus far.
My right arm still bothers me, especially if I type for a long time. Frequent breaks make it better, but I could definitely never play computer games for long stretches any more (that's why I switched to console games, although even those hurt after some time). If I even touch a mouse with my right hand, the pain almost immediately starts. It's no good.
So now people say, "Hey, you're left handed!" when they see me using my computer. It's a bit ironic, since I've always been left handed. I would punch these people, but my right arm hurts too much.