Should I turn my computer off at night to save energy? Or does turning it off every night wear out the hardware?

The main thing that wears out is the hard drive. However, it turns out that warming and cooling of the drive can cause more wear and tear than just leaving it running all the time.

What I do is have my computer go into standby after 2 hours of inactivity. This will run the CPU at a slower clock speed (using less power) and also spin down the hard drives. I decided I'd risk the thermal changes in the interest of energy conservation.

Others may chime in with additional/different ideas.
WallyH

I agree with Wally, good suggestions.

Another reasoning is that electronic devices are under the most "stress" at the moment they are first turned on and energy infuses the system... (Sort of like a lightbulb burning out when you turn it on... )

Anyway, we only turn off computers when we are going to be gone more than 24 hours.

I leave mine running 24/7 on a UPS and conserve elsewhere - switched to LCD monitor and compact flourescents, programmable thermostat, keeping refrigerator coils clean, etc. Computers don't use that much power. You'll gain more energy savings from ensuring your high consumption devices are operating the most efficiently than you'll ever get from giving the computer a few hours of off time.

I also have a WattsUp power meter I've found really handy.

Just chiming in. I leave mine on too most of the time. I have my monitor go to sleep after 1 hour and the hard drives turn off after 45 min. (Don't ask me why I picked those settings. I guess cause if I actually go that long w/out touching anything, I'm probably gone or really busy. :-)) Occasionally I turn it off at night cause it makes me feel better to give it a full rest occasionally.