[Watabou]

say is pretty useful and fun. When I'm compiling or just telling the Terminal to do something tedious, I add a say command at the end and it will notify me when it's done.

For example, if I want to install mercurial and git with homebrew:

brew install git mercurial; say "The task is finished, Master."

[Tarlitz]

A more silent option is to print the bell character to the terminal, ie.: sleep 5; printf '\a'

This will cause the terminal to bounce and add a little badge with the number of tasks that require your attention if it's out of focus :)

[jasomill]

As an aside, in Lion at least, you can download high-quality voices for dozens of languages in Speech Preferences (including the ones Apple uses for Siri in several countries).

[riobard]

I used to SSH into my wife's Mac and asked her to come over if we are in different rooms :)