From Gmail Notifier Pro:
Gmail Notifier Pro Portable is a professional Gmail notifier for Windows supporting multiple Gmail accounts. The application will display notifications when new messages arrive. It contains many easy-to-use features and settings like customizable themes, individual account settings and several options for how notifications are displayed.
Gmail Notifier Pro Portable gives a complete overview of all unread mail in all your mail boxes and support secure Atom and IMAP protocols for mail retrieval. It will also let you write and reply to mail without opening your browser, and can be registered as your default mail handler. It also includes integration with Google Contacts, scheduling, can run from USB and supports automatic updates. This ZIP archive is for running Gmail Notifier Pro without installation.