*Amendment - thanks to those of you who commented that you require Exchange Server 2007 to take advantage of the functionality I discuss in Outlook 2007*
I'm about to head out on holiday for a couple of weeks and have used the Out of Office feature of Microsoft Office Outlook 2007. It's hardly a new feature but it's implementation is so much better in this version than previously - as you can see below there are far more useful options:
I really like the "only send during this time/date range" as I can set up the Out of Office (OOF) when I remember rather than as the last thing I do before leaving. I also like the nice clear way I can set the message for "inside my organisation" and "outside my organisation"