http://www.dansdata.com/sbs22.htm
Some people leave their computers on as long as possible, on the grounds that components come under the most stress when turned off and on, and so you're less likely to have failures if you leave the power switch alone. There's only a grain of truth to this.
Rapidly cycling the power - working the power switch as if you're being paid by the click - is a bad idea for many electronic devices, including computers and monitors. If you're talking about ordinary use, however, the only problems you're likely to encounter stem from differential thermal expansion. Things get bigger as they warm up and smaller when they cool, and different components in a computer expand and contract by different amounts. The resultant mechanical stress can, theoretically at least, break traces on circuit boards and cause similar havoc.
In the real world, thermal problems with personal computers practically never have anything to do with differential expansion, but instead stem from lousy ventilation. Hot components, hard drives in particular, can barbecue themselves into an early grave. But these failures happen pretty seldom, these days, and modern hard drives are very unlikely to suffer motor or solenoid failure on startup. An old drive that's developed "stiction", where the drive has a hard time spinning up, should be left running all of the time. But that problem's never been common and is now close to unknown.
Current "green" PCs with power management features let you have most of the convenience of an always-on PC without the power bills; you can put your monitor, your hard drives, your processor and even the whole computer to sleep, and wake everything back up in a few seconds. From a differential expansion standpoint, this is the same as manually powering off the components in question. But since differential expansion is unlikely to ever do anything bad to your computer, who cares?