softraid(4) is a little further along than jggimi mentions, it is in the default kernel after all.. unfortunately the major problem is that backwards compatibility is not a goal, and newer viewers may break on-disk structure (..forcing you to rebuild the array).
A bonus with softraid is that it is controlled and monitored with the same utilities as many of the hardware RAID controllers,
bioctl(8).
There are 4 methods available,
ccd(4)..
raid(4) - (..aka RAIDframe)..
softraid(4).. and hardware RAID controllers (..
apropos RAID).
Good luck.