GMail Drive is a Shell Namespace Extension that creates a virtual filesystem around your Google Gmail account, allowing you to use Gmail as a storage medium. GMail Drive creates a virtual filesystem on top of your Google Gmail account and enables you to save and retrieve files stored on your Gmail account directly from inside Windows Explorer. GMail Drive literally adds a new drive to your computer under the My Computer folder, where you can create new folders, copy and drag'n'drop files to. Ever since Google started to offer users a Gmail e-mail account, which includes storage space of 2000 megabytes, you have had plenty of storage space but not a lot to fill it up with. With GMail Drive you can easily copy files to your Gmail account and retrieve them again.
Please note that GMail Drive is still an experimental tool. There's still a number of limitations of the file-system (such as total filename size must be less than 65 characters). Since the tool hooks up with the free Gmail Service provided by Google, changes in the Gmail system may break the tool's ability to function. I cannot guarantee that files stored in this manner will be accessible in the future.
8 Nov 2007 update:
Google is preparing for Gmail 2.0 and restructured their login. They will allow 3rd party tools to access an interim version for some time, but be prepared that GMail Drive may break again. Version 1.0.12 fixes the login, and now works with Google Apps accounts (hosted domains) too.