Google has released a new (experimental) feature to Gmail that allows you to cache your emails and access them offline. It also lets you send and respond to emails while offline and automatically sends them when an internet connection is present.
I know a lot of people have been waiting for this feature for a while, myself included. This is a big step toward being a serious competitor to email applications such as Outlook, Thunderbird, etc, and could pull over a lot of the people who are still sticking with traditional email clients.