What's the Linux equivalent of OS X's Growl?

Ubuntu can show notifications, which seem to be equivalent to Growl. It is accessible through DBus (for programs) and through the notify-send command (for shell scripts).

There is a Growl port for Linux. Found here: Growl-for-linux

if you are using KDE, it is real easy! just use the kdialog command that is built-in to KDE, as in the example below:

kdialog --passivepopup "Example text"

if you happen to be using plain ubuntu, then you need to install the libnotify-bin package by issuing the following command:

sudo apt-get install libnotify-bin

and then you can send messages with like this:

notify-send "Example text"

the kdialog is a bit more interesting because it does way more than just send notifications from the command-line. you can actually build an interactive gui for your bash programs with it!

anyway, i hope this helps.