As RichardLetts already surmised this is Growl for Windows running on my desktop. I use the execute a script action in alerts to pass variables to a batch file that then executes a small program called growlnotify that sends the notifications to the desktop application. You can then let other users install Growl for Windows and subscribe to these notifications on the desktop receiving them so you don't have to send the notifications to each user via the script.
I will write up a quick tutorial on how I accomplished this soon.
Cheers