GmailWatcher is a Gmail notifier especially designed for Ubuntu. It supports multiple accounts, labels (you can select for which labels to receive notifications) and comes with Ubuntu Messaging Menu integration / NotifyOSD support.
The application displays the unread email count in the Ubuntu Messaging Menu and clicking it, you can see details such as the sender, title and so on, each under its label - all without having to open your browser.
For the past few weeks, Owais Lone, the GmailWatcher developer has been rewriting the application which now supports GTK3 and comes with a new user interface (see screenshot above).
Besides the UI, GmailWatcher also got some new features like instant notifications, easy account switching, displays unread email count on the Unity launcher (only when opening the main GmailWatcher window - you can pin it to the launcher to always see the number of unread emails) and more.
The new GmailWatcher is not yet stable and is only available for Ubuntu 11.10 Oneiric Ocelot for now. If you're already using Oneiric and want to try one of the best Gmail notifiers for Ubuntu, install it using the commands below:
sudo add-apt-repository ppa:loneowais/gmailwatcher.dev sudo apt-get update sudo apt-get install gmailwatcher
Report any bugs you may find @ Launchpad.
Thanks to Owais for the tip!