Is there something (free) like ZFS or HadoopFS or GFS for Windows?
Solution 1:
AFS might work, but it's still more centralized than you're likely to want to deal with. Read more about it at http://www.openafs.org/windows.html.
Solution 2:
What about Windows Distributed File System? That should allow you to create one drive that is in reality a mixture of multiple partitions across different disks/servers
Solution 3:
Plan 9 from Bell Labs?
I asked a similar question on ServerFault recently: Is there a way to do something like LVM over NFS?.
Solutions suggested there were Lustre and GlusterFS.
Solution 4:
What you are asking for reminds me of Wuala (Google Talk video), except to be run in-house and not via the web. In the talk some people mentioned other technologies.
I too love ZFS and was waiting for MS to make Windows compatible with it, clone it or come out with their own (something like zNTFS).
EDITED: I found some software I tried a long time ago. It isn't a way to store files like a file system, but it is a distributed backup program. The program is called Vembu StoreGrid Backup Software. I'll keep looking for something that is more file system like.
EDITED: xFS shows promise, but looks to be research.
The only thing better than using unutilized space on the network is using ZFS on it.
Good luck.