I consider myself very much a righty, however...
I did teach myself to write lefty in school. If I don't concentrate though, I end up mirror writing instead, and the mirror writing is way neater than the normal way.
I worked 20 years as an OR nurse, a certain amount of ambidextrousness (is that a word?) is not only handy, but in some cases necessary. Holding a retractor in one hand and snipping sutures with the other was normal. Third or fourth hands would have been advantageous!
I use the phone lefty. I cannot bring myself to hold the phone to my right ear at all.
I hold jars with my right hand and open the lid with the left. Don't know why, and it's a pain as my left is definitely weaker than my right.
These days I have carpal tunnel in the right hand, and arthritis in my left thumb, so I tend to switch things around a bit as required, particularly things like using screwdrivers, drills, saws etc. Handy!
My car is manual (stick shift) and I'm in Aus, so it's left hand for the gear stick, but that's what we have here, it's all I've ever known. It would feel weird to shift gears with the right hand. The motorcycle, fortunately, seems to be set up the same over the world (except for odd makes). Maybe next time I'm in the US I'll rent a bike instead! :-)
I definitely crochet and knit right handed.
And every now and then, when I grab a pen to write something, I grab it with my left hand for some reason. It's not till I'm trying to work out why it feels wrong that I realise I've got it in the 'wrong' hand.