Do you ever turn off your computer?

what's this "off" you are talking about? :)
if you have a computer from the last 2-3 generations, idle power usage will be quite small so it won't be too bad on the electricity bill. i've spent some money and effort on getting my computer to be quiet if not silent and at idle it's effectively silent (and fortunately i sleep in another room).
reasons? sometimes downloads, letting dd platforms do their upgrades (steam, origin, bnet, desura are running pretty much all the time), irc, now less important but it used to be good to be online for im-s as well. besides, it takes a while to start the computer up, with all the stuff that gets loaded, a minute or two.
convenience, not necessity.

Everytime I'm not sitting in front of it, except there are some downloads/other stuff I want to finish.
Doesn't make sense for me, to let it on, when it's not in use, I also can live with ~15s startup time. :)
Even, if restarting a PC would wear it off noticeably, I would still turn it off, because in the end, I think, there wouldn't be any difference on my budget.

Every day unless I am downloading something

Never. If I did Skynet might win.

If I am downloading games, I leave it on. Otherwise it gets shut down. The only way I see that turning off your PC can cause damage is if you turn it off and on constantly. It's best to just leave it on for most of the day and shut it down when you're sleeping or just won't use it for most of the day.

i turn off, making ur computer on 24/7 will reduce his lifetime a lot

Never no ways! :D

I turn it off when I go to sleep because I need my rest and computer can be too noisy. Once upon a time it was turned on for a year without turning off or restarting (I was using GNU/Linux back then).

i prefer to use sleep instead of full shut down.

turn it off. saving your HDD life cycle.

yes because my computer consumed a lot of power and im not a millionaire...

the only reason for letting it on is when i download something over night.

Its good for your pc to turn it off now and then. All the temp cookies and other processes in the background pile up, and slow down your pc. I turn it off when im not gonna download something over night. I usually just shut it down.

I never turn mine off, maybe restart once every month-2months if I have to, it seems happy enough.

Of course I turn it off! I always do it every night and when I'm not using it. Why would I keep it on when I'm not?
It saves energy, money in my electric bill, and expand the lifetime of my hardware.

Only when I go out or when I go to sleep.

My computer is never off, unless the power goes out, or I need to fix something.

I put it in standby mode if I won't use it in the next 15-30 minutes and turn it off if it's longer.
I don't like to have a computer running if i'm not using it, I don't know if it will expand its lifetime to never turn it off, but I don't see any logic in doing that.

Only time i turn mine off is when i need to clean out dust or fiddle around with something inside or if i wont be using my pc for more than a day.

The whole turning on and off damaging the pc is about stuff getting hot and cold alot causing it to expand and contract which can compromise the solders on your hardware. But all it really boils down to is the cost of having it running all the time or not which isn't all that much, specially if you have stuff go into low power mode when not being used.

I always turn mine off when I go to sleep.

yes

I only leave it on if I have a reason for it. Usually I don't

For the people claiming the heating up and cooling down damages the parts: don't buy cheap parts. The only hardware I ever seen break in my life is a single videocard that fried in a power surge (no surge protection on that pc at the time) I have to replace parts because they are outdated long before they break.

Plus, usage probably causes more temp fluctuation than turning it off. (unless so have insanely good cooling or you constantly run your parts under the same load)
My videocard idles at 35C, room temp is around 20. So the same thermal stress is achieved if my GPU reaches 50C even only once during the day. I believe these kind of videocards have the safety shutdown at 100C (for the ones not knowing the C scale well)

Yes, every night, or when I leave my house.

Even with my old computer, but with my new one, it doesn't even matter because it takes about 15-20 seconds to start with my SSD anyway.

I don't turn mine off unless I have to... I'm under the impression it's easier on the hard drive/s to just leave them idle, rather than turning them off and on all the time.

Of course I do, I'm not a millionaire.