And what if that program you want to make growl isn’t running on a Mac, but you’re parked in front of one? Well then, you need Eric Hodel's ruby-growl:

Growl’s existing Ruby bindings only work on Macs, and I wanted my non-Mac machines to be able to send Growl notifications. I peeked inside Growl’s remote-notification capabilities and wrote up some code that uses UDP to send Growl notifications across the network.

With ruby-growl you get a Ruby library you can use to send Growl notifications from within your Ruby applications and a command-line tool similar to growlnotify.

Grr!