This Page by Mark Lyon is full of useful GMail tools. Including importer, exporters and migrators, as well as POP3 access. Looks like a must have for GMail users.
Update: Get the GMail Helper direct from Google. It adds system tray notification and resolution of mailto links to most any Microsoft Windows system. No official support that I know of for Mac or Linux yet. I also hear that some of those unofficial tools from the previous link are being blocked by Google.