Outlook 2007/2010 only: Few things make work feel like a moment from Office Space more than seeing an email hit an entire department's inboxes, over and over again. If you're in an Outlook environment, the NoReplyAll add-on stifles the ad nauseum action.
The NoReplyAll plug-in simply adds two buttons to your composition window in Outlook: "No Reply-All" and "No Forward". They do exactly what you'd think, at least among your fellow Outlook recipients. The reply-all and forward buttons are greyed out when they open your message, and while still technically possible to send a message back to everyone in the To and CC fields, it would be a copy-paste pain.
NoReplyAll is a free add-in for Outlook 2007 and 2010. It's highly recommended for birthday announcements, lunch requests and other situations where you're likely to inadvertently give everyone in the office a megaphone to air their personal items of interest.