Windows only: Scott's Gmail Alert 4.0 is a feature-packed Google Calendar and Gmail notification tool, offering great looks and a wide array of customisable options, including the ability to only schedule notifications only during pre-defined times. Distractions begone!
In addition to Gmail and Google Calendar support, Scott's Gmail Alert also includes the ability to watch RSS feeds. Everything from the location, the colour and the opacity of the alerts to how frequently the application pings Google is customisable.
Another little nifty feature is the ability to quickly reply to or send an email without opening up your browser (you can even set the app as your default mailto handler).
Additionally, if you have another application running full-screen, you can choose not to display alerts at all. The application is free, available for Windows only.