Many have mentioned an amusing hack that allows you to use your 1G of GMail storage as a mountable file system (for you non-geeks, essentially another drive visible on your desktop). Heh, heh. Very geeky, though totally impractical.
In speculating about Google's next moves, some have talked about Google's potential as massive remote storage system. The idea is that users could store data remotely and replicated across the Google cluster, providing a service similar to iBackup, Xdrive, or Yahoo Briefcase. I'm not sure about this one. Google's cluster is heavy on CPU and memory and light on disk, perfect for operating over distributed search indexes, but probably not what you want for a simple remote storage system.
However, the Google cluster may be a good platform for remote services and applications, since this would take advantage of the available computing power.