GMail introduced a new feature recently, that is browsing mails offline.
First you need to install Google Gears, you can download the online installer from here
http://gears.google.com/
Run the online installer and it will install Google Gears on your system.
Thats it you are almost done, just a few clicks away. It will ask your permission to install offline access for GMail, click next.Then you get a Google Gears security warning, allow it.Click ok in the next window and you are done.It will download all your mails to your hard drive for offline browsing. You can see the status in the status window.