Sheepdog & Ceph/Rados provide a means to have distributed block devices. Right now, Sheepdog is only supported as a storage driver for Qemu.
http://www.osrg.net/accord/ - a high performance coordination service, similar to Zookeeper.
https://bitbucket.org/hirofuchi/xnbd/wiki/Home - XNBD & links to NBD projects.