Spiffy is a stand alone Gmail notifier for Windows, it's free and totally customizable.

Features

Support for 5 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

Proxy support (supports authentication)

Runs in System Tray

Autostart & Hide options

Works from USB drive

Requires .NET Framework 2.0+ and a Gmail account.