Imagine you have a machine with all of its disk full and another with unused gigabytes, and you don't want to move the data from one to the other.
Read it at howtoforge
Why not using the second's disk on the first, you can do it with iSCSI but you can do it with ATA over Ethernet (AoE) too. It's the second method I'll explain in this article. All of this was made with two computers running Debian Etch.