With the release of Leopard Apple also introduced us to Alex, a new "voice" for text to speech that is extremely life like. When Alex speaks the voice introduces breaths in between sentences and has incredible inflection. Though it still sounds like a computer it is a dramatic improvement over the older voices that have been out there.
You can use the Keyboard Shortcuts feature to have your Mac simply read any text you highlight, which for users that don't have an accessibility need can use for entertainment. My favorite use is to read acerbic comments from people in blogs. There's something very funny about hearing a computer voice go through a rant on why they love or hate Macs or Windows.
Also, Growl can create events that use the text to speech feature, as can products like Adium (which also can use Growl).
If you haven't heard Alex speak yet go into your Preferences / Speech window, change the System Voice to Alex and hit play. It's pretty cool.