If you are searching for a nice system wide software to display notifications for Windows (like Growl for Mac) you will find Snarl (www.fullphat.net).
Starting today we have now a fourth extension in our project: SunSnarl 0.1.1 displays notifications fur reminders in Mozilla calendar application Sunbird. It can be configured to display the standard reminder (incl. options to snooze) and/or the Snarl notification.
Also available in this project is a Snarl-extension for Thunderbird called Thundersnarl (for notifications about new mails), an extension called FireSnarl for Firefox (which alerts about the states of downloads and captures all standard alerts) and an extension called PidginSnarl for Pidgin (which notifies about chat messages and buddys signing on and off as well as new mails in your GMail-Inbox).