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. LINK
Peer2Mail is the first software that let you store and share files on any web-mail account. If you have a web mail account with large storage space, you can use P2M to store files on it. Web-mail providers such as Gmail (Google Mail), Walla!, Yahoo and more, provide storage space that ranges from 100MB to 3GB.
LINK
I have tried neither one, but they sound like good ideas.