Windows only: System tray utility Gmail Notifier tells you when you have new email messages, allows you to check multiple accounts, and lets you read, delete, or mark the messages as read right from the notification utility.
Once you've installed the utility, made sure that IMAP is enabled in Gmail, and added your Gmail account, you can access the preferences from the tray icon to immediately turn off the rather annoying notification voice or add a new account. The main notifier window shows a list of your accounts, and allows you to read unread messages, mark them as read, or delete them. Since the notifier uses IMAP, the changes are reflected immediately on the site.
It's not a perfect notifier, with some odd UI quirks—for instance, the minimise button turns the window into a search box—and the actual notifications consist of a plain tray icon alert instead of the slick notifications we're used to in other products. The interesting piece is the ability to quickly preview messages and mark them as read right from the interface itself, which could be useful for keeping your inbox clean. Gmail Notifier is a free download for Windows only.