This one’s been around for a while (I imagine) - a tool that maps a “drive” (like C:\) on your Windows PC to your Gmail account, allowing you to directly access the file storage capabilities:

Since it’s not an official feature of Gmail, Google could decide to retract use at any time, and since the files aren’t stored as emails, you might lose access to them. Use at your own risk ;)

Download link: http://www.viksoe.dk/code/gmail.htm

Picture credit: viksoe.dk