You can use it to display local notifications from the Plex machine, or even use it on a remote machine to send it's notifications to your Plex system.
You can download it here: http://themacbox.co.uk/stuff/XBMC.growlView.zip
Here's what notifications look like if you haven't seen one before:
Here's how to configure the plugin:
1. If you don't have the web server enabled, enable it in Plex > Preferences > Network > Enable web server. While you're there, make a note of the port, and set the password if you plan on making it outside accessible.
2. Install the growl plugin and open it's settings.
3. If you have a password set then enter "plex" in the username box, otherwise leave it blank.
4. If you have a password, enter it in the password box.
5. Enter the IP or hostname of your plex system, you'll need a static IP if you're sending to a remote plex server, if you want it externally accessible use something like DynDNS and forward the web interface port to your plex system. If you're just displaying local notifications you can just enter localhost. You also need to enter the port as shown in the screenshot. So if the IP address is 192.168.1.199 and the port is 3000 (the default) then enter 192.168.1.199:3000 or if you're using localhost then enter localhost:3000 etc.
6. Click verify and if your plex system is running and the details are correct it should say "Success!"
7. You can enter the number of seconds you want the notification to display for if you want, and also choose another display plugin if you want it to display as a normal growl notification as well (eg. for when Plex isn't running)
If you have any problems with the plugin, don't hesitate to let me know and I'll see what I can do. :)
Oh, and say thanks if you like it!