git clone git://gitorious.org/ransrid/ransrid.git
Unfortunately, there is little (read: no) documentation available yet; that will change after LinuxTag. Upto then the only doc is the heavily commented source code. Grab it, study it, enhance it, send a patch - that's the open source way.
For LinuxTag I have another goodie - I will be traveling with four USB disks and give a short live demo of what the system is already capable of. Live add and removal of disks isn't working yet, but reading, writing, validation, and rebuilding is.
Note that nbd used to freeze machines during writes if client and server were running on the same machine. Since kernel 2.6.26 there is a patch included that ought to fix this issue, but there were some (inconclusive?) discussions about this patch beforehand. Using xen for the client seems to work around this issue as well, though.