Spiffy is a Gmail notifier for Windows, it's free and totally customizable. Check it out...
Features
* Support for 2 Gmail accounts (inbox or custom label)
* Skinnable tray popup alerts on new e-mail (optional)
* Tray icon change on new e-mail (optional)
* Sound alert on new e-mail (optional, mp3, wav)
* Set custom 'check for mail' interval (all accounts)
* Show alerts once or until read in Gmail
* Tell Me Again feature for missed alerts
* Proxy support (supports authentication)
* Customizable GUI
* Runs in System Tray

The license of this software is Freeware, you can free download and free use this email checker software.