Google Gears is a great way to access web content offline. Google Services such as Google Reader and Google Calendar made it possible to download feeds and tasks on the computer and access them later from the same web interface when offline. Even third party services like Remember The Milk offered support for Google Gears. However, Google is now offering Gears support for GMail.
Before, it was only possible to access mails offline by fetching them by POP3 or IMAP. But now you can use Google Gears and use the GMail interface to browse mails when offline. This feature is currently experimental and can be enabled through the Google Labs tab.