Sleep your Mac differently depending on the battery level.
Your MacBook or MacBook Pro knows the following sleep states:
sleep machine will go to sleep only (saves state in RAM only, battery keeps RAM contents)
hibernate only machine will go to hibernate only. (saves state on disk, battery will not be used)
sleep & hibernate machine sleeps and hibernates. (default)
Just sleep means that the notebook will go to sleep fast, but you lose the ability to change the battery as the battery is needed to keep the contents of the memory (RAM).
Just sleep and hibernate will wake the computer fast, but sleeping will take ages as the contents of the memory are saved to disk before entering the sleep.
SmartSleep lets you select each sleep state. Additionally the new SmartSleep state lets your notebook just sleep while the battery has a high level. If the battery level drops below a certain point (default is less then 20% or 20 minutes) it will switch to sleep and hibernate. So you have the best of both worlds. Furthermore it will expand the lifetime of your SSD (if you have one) as your MacBook won't write a hibernate file as often.
Mac OS X 10.6 Intel