I've been using this package as a notification system on Macs for several years now and it works well for our needs.

I find Growl to slow start up times on my systems and do not use it, but wanted similar functionality for notifications in an App we were distributing.  Also not everyone has Growl and this guarantees the available functionality in our App.

To use it:
package require tkgrowl
::tkgrowl::Growl "Title" "Message"

This creates its own preferences folder and file and on first launch shows the preferences pane to users.
You can call the preferences pane later by adding a menu item to your App that calls ::tkgrowl::Display_Preferences

I currently do not have a Windows box to test against however it should theoretically work correctly.

If anyone would like to test it and contribute any fixes or feedback that would be welcome.

Standard BSD license applies and is included in the download.
[ Around 8k zipped and 37k unzipped ]