In certain countries, GMail is offering an option to enable GMail usage when you are not connected to the internet. It works using Google Gears and by downloading all of your mail directly to your computer. You can then use all of the features of GMail at any time, even when you have no connection. Obviously new mail can not be sent or received when you are not connected. But GMail will hold it all in reserve and as soon as you have a connection, it'll send out mail as needed and synchronize your folders.

This new offline functionality in GMail is not on by default, you need to turn it on. You do this by clicking the Settings link in GMail (in the upper right corner of the page). Then click on the Labs tab in Settings. You'll see a section called Offline. You need to select the Enable radio button and then go to the bottom of the page and click the Save Changes' button. Once you do this you'll get a new Offline link in the upper-right of your Gmail Inbox.