GmailFS

The first real reason to make me want Gmail: GmailFS.

"GmailFS provides a mountable Linux filesystem which uses your Gmail account as its storage medium."

A Gmail account + GmailFS could join tools like Nmap in being one of the few indispensable tools for a hacker. Pop on any box locally or remotely, grab GmailFS, and mount your Gmail account; all of your tools now sit at your fingertips.