Windows only: System tray notification application Google Reader Notifier for Windows tells you when there are unread items in your favourite RSS reader.
The application is easy enough to use—just plug your Google account information into the settings dialog, and you'll see a small popup dialog whenever there are new items to read. You can choose how often the notification happens, or even turn off the popup altogether and simply use the tray icon indicator instead. Since the application first appeared years ago a real programmer has finally taken over the project and added audio notifications, filtering by multiple tags, and even support for Snarl notifications instead of the built-in variety.
Disclaimer: the original semi-working application was written and then abandoned years ago by, well, yours truly. It's a testament to the power of open source that somebody else has actively started developing the project and turned it into something better.
Google Reader Notifier for Windows is free and open source.
Google Reader Notifier for Windows [Google Code]