mdadm is a tool for managing Linux Software RAID arrays.
It can create, assemble, report on, and monitor arrays.
It can also move spares between raid arrays when needed.
It can be found at http://www.kernel.org/pub/linux/utils/raid/mdadm/ or any of the kernel.org mirrors.
There is a git repository at http://neil.brown.name/git/mdadm or git://neil.brown.name/mdadm.
FAQ and other documentation can be found at http://www.linuxfoundation.org/collaborate/workgroups/linux-raid