Mac OS X has supported for a very long time putting Macs to sleep. This is a must-have feature for laptops, but is also convenient for desktop machines. However, it hasn't been since the transition to Intel-based Macs that it also supports hibernation, also called deep sleep. When entering the hibernation mode, the system stores all memory contents to disk as well as the status of the devices. It then powers off the machine completely. Later on, the on-disk copy is used to restore the machine to its previous state when it is powered on. It takes longer than resuming from sleep status, but all your applications will be there as you left them.
Now, every time you put your Intel Mac to sleep it is also preparing itself to hibernate. This is why Intel Macs take longer than PowerPC-based ones to enter the sleep mode. This way, if the machine's battery drains completely in the case of notebooks, or the machine is unplugged in the case of desktops, the machine will be able to quickly recover itself to a safe state and you won't lose data.
As I mentioned yesterday, I've been running my MacBook Pro for a while without the battery, so I had an easy chance to experiment hibernation. And it's marvelous. No flaws so far.
The thing is that I always powered down my Mac at night. The reason is that putting it to sleep during the whole night consumed few but enough battery to require a recharge next morning to bring it back to 100%, so I didn't do it. But now I usually put it to hibernate; this way, on the next boot, I can continue work straight from where I left it and I don't have to restart any applications.
Now... putting a Mac notebook into this mode is painful if you have to remove the battery every time to force it to enter hibernation mode, and unfortunately Mac OS X does not have any "Hibernate" option. But... there is this sweet DashBoard widget called Deep Sleep that lets you do exactly that! No more boots from cold state any more :-)