Rob’s Blog has the definitive list of both official and third-party tools for GMail, including some very useful add-ons. Here’s the best of the bunch, just to save you time:

• GMailIt - sets your default mail program to GMail, meaning all mailto: links on webpages will link you straight to GMail’s compose page, with the To field already filled in.
• GMail drive shell extension - allows you to access your whopping 2GB space via Windows Explorer, just like a normal hard drive partition. Veeeery useful for quick backups.
• Icon generator - download an image icon of your GMail email address for putting on your website or as a forum signature. Stops the spammers nicely.
• Official notifier - adds an icon to the system tray and alerts you when you have new email messages.

GMail Tools & Plugins (via Oishii)