Safe Sleep is basically a 'deeper sleep' feature that debuted on the 15-inch and 17-inch PowerBooks last year, and as far as I know, it's included (and enabled by default) on all of Apple's MacBook/Pro portables (and yes, I hear it's basically like the Windows 'Hibernate' state). This deep sleep differs from the typical Sleep state that most of us are used to: it writes everything currently in memory (your open apps, the files you're working on, etc.) to the hard drive, and actually powers down the machine, saving the battery power that slowly siphons during the normal Sleep mode. Waking from this mode naturally takes a little longer, and a progress bar unique to this feature is displayed while the machine is waking from Safe Sleep. If you want to see it in action, Rob Griffiths (of Macworld and Mac OS X Hints fame) posted a good video demonstration of Safe Sleep on his black MacBook (so jealous!) to YouTube.
By default, the new MacBook/Pro behavior is to use a little of both worlds: they write everything to the drive while they're going to sleep, but still use 'regular Sleep' unless power is lost from both AC and the battery. If power is lost, the machine switches over to Safe Sleep automatically - yet another reason to send those Apple engineers a batch of cookies. But what if you want to bypass the regular Sleep status and use Safe Sleep by default all the time?
Enter the Deep Sleep Dashboard widget - one-click access to putting your Mac down for an extended nap. Documentation is included with the widget explaining what's going on, and it also has a list of the machines known to support this feature. To top things off, if you just aren't a fan of widgets, the author also packaged Deep Sleep as a simple command line utility, linked in small print at the bottom of Deep Sleep's page.