> Can someone tell me if it's best to leave your computer on all the time or shut
> it down when you're not using it?
I assume that by "best" you mean "best for the life of the equipment".
If everything is properly cooled and grounded (and I mean everything: cpu, mobo, drives, cards, psu, whatever), there should be no big threat in leaving it on. Moreover, in general the MTBF (Mean Time Between Failure) of hardware working at constant temperatures and conditions is longer than the one of devices being switched on/off over short periods.
> Does leaving it on all the time lower your
> hard drive's life span?
Au contraire. What really kills a hard drive is the spinup/spindown that happens when you turn the pc on/off. A disk that constantly turns, if properly cooled (did I mention that you have to cool it down?) could even last longer.
Obviously, a bad hardware would make any caution futile, together with the assumption that this rule is valid for hardware designed to be frequently switched on/off and stay on just for a few hours. I wouldn't leave a laptop turned on forever.