MDADM is a Linux tool used to create, manage and monitor sofware RAID arrays (RAID 0, 1, 4, 5, 6 and 10). It's a free tool that can also be used aside from Linux, like raidtools developed by SUSE.
It also enables particular non-raid situations like Multipath and Failover with different "voluems" on the same physical disk in order to simulate fault situations.