my machines are powered on only when I use them, when they're not in use they're turned off.
Saves money on my electricity bill, reduces pollution, reduces wear & tear on machines.
Turning computers on/off doesn't hurt them from what I know - been in this "business" for almost 20 years. I have however seen power supplies that have failed on units that are left on all the time, my brother-in-law used to leave his desktop pc on all the time until his power supply failed and started to smoke up & spark for some reason, filled the basement recroom with smoke, my in-laws called me over, they were afraid to turn it off because it was making weird noises and filling up the basement with smoke. Suffice it say by the time I got there, the damage was done: the smoking power supply was still providing some form of partial power to the pc and in turn smoked the mb, memory, harddrive, video, audio, network cards, etc. When we powered off the machine (and aired out the recroom), I opened up the box to reveal a dusty machine that was covered in dust & pet hair. The pet hair may not be a standard thing in most homes but dust is inevitable and leaving these machines on 24/7 just compounds the problem. Judging from the bulk of machines I've ever had the "pleasure" to service, most people never dust out their machines - leaving them on all the time turns them into dust magnets. The build-up of dust & dirt in these machines has detrimental effects: dirt & dust (& whatever else is floating in the air) constantly gets sucked inside the case of your pc - and eventually clogs both the pc power supply and fans. As it accumulates, the fans have to work harder to perform the same job, after a while the fans can slow down or fail completely which leads to overheating inside the case (your pc's cpu runs slower as it runs hotter), overheating also can cause other hardware failure and can lead to data loss, dust building up on your motherboard & other hardware components can build up a static electricity charge and one random discharge will fry your motherboard and probably other components as well (have you backed up your data lately?).

You can alleviate these problems by regular dusting the interior of your pc with compressed air (you can by a can of compressed air for pc cleaning for $5-$10 from your local pc shop) and turning off your computer when it's not in use.

You could also be one of those lucky people who leave their pc's on 24/7 and never ever experience any problems (except for above average electricity bills).   

on

On during day, sleep at night, or when I know I won't be using it for a while, reboted once a week.

 Coincidentally, on Yahoo front page almost as soon as i posted... WWW Link   

i turn mying off when i'm thru playing it.frankly people will tell you that it will make the MB warp and eventually burn out..honestly it's false i've been doing it for years and yet have to replace my MB and other hardware..if want to get technical bout it when the system is constantly on (and this was on the discovery channel bout this ) you get what they call "whiskers" this is when the solder (or something like that) is subjected to heat for such a long period of time that these "whiskers" grow really long and they will eventually touch each other and when that happen BAM!! the systems piece of hardware will fry causing you to replace that piece of hardware...also note these "whiskers" was also responsible for the system malfunction of some satellite (i forget the satellites name) and just incase you don't believe me WHISKERS

I have mine on a special power-connector so that everything else not needed (printer/monitor/surround-speakers...) is turned off automatically also when I shut down the main unit and of course also turned on when I start the PC via the main button.. So off here during the day when at work and the night.

Sooner or later some propeller-hat-wearing Linux freak will pop in to tell you a 'Nix box has been running without restart/power-up for two hundred and seventyfive years.

He'll be lying, of course.

It was longer than that.

Good practise would have you turning them off when not in 'use'.   I tend to ignore good practise....far too dull and boring...so mine's on 24/7 for around 350 per year.  I turn it off when I'm away at Phillip Island for the MotoGP and World Superbikes and V8 Supercars....but otherwise it's on...

Off when not in use....see no sense to have it on.

That was a good article, but reading the posts....  They all say different things too...

Off when not in use. No sense in wasting electricity, no matter how little it may be.

I turn my main computers off, but the server has to stay on for obvious reasons. It is an old Gateway from 1999 so I'm not too worried about the power it sucks up.

I turn mine off when I am not using it
Yeah I agree there is no need to waste electricity and systems for leaving it ON on the time.
NO turning a system off does NOT Harm it..

Turning a system off does not harm it, but when you turn it back on, the hard disk has to spin up again, which can start to wear down the motor.

Always on , I'm a t.v. series downloading addict and when you shut down in the middle of a download sometimes it will not start up again.(Usually have about 30 going at once)Plus I have 4 fans to keep it cool and one on my videocard.

I keep my computer off when I'm not using it because it is a waste of electricity, and it needs reboots anyways to clear 'leaked' memory from programs. Also, the fans are rather loud . And the fact I have a 600 Watt PSU, just chopping money out of my wallet.