[From the Homepage]
SafeSleep... Mac OS X 10.4 improved a little-known feature of Mac OS X; in a low-power situation the contents of memory will be saved to the hard drive and the computer will shut down. On other Operating Systems this is a standard feature known as Hibernation or Suspend-to-disk.
SafeSleep lets you quickly and easily put your laptop into this sleep mode on demand. I have been using it for several months without any problems. But note that this usage of the functionality is not officially supported by Apple.
Requires: Mac laptop, Intel iMac, MacOS X 10.4 or later