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).
FireSnarl is an extension for Firefox (which alerts about the states of downloads and captures all standard alerts), SunSnarl is an extension for Sunbird (displays reminders) and ThunderSnarl is an extension for Thunderbird which displays new mail notifications.
All three extensions are now equipped with a new internal interface which provides much more possibilities for enhancements in the future. Already included starting today are translations in FireSnarl (English, German, Polish, French, Portugese and Japanese) and SunSnarl (English and German) and in ThunderSnarl the display of special characters in subject or names is now correct (translations also to follow for ThunderSnarl with the next release).
Also available in this project is 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).