I just use the built-in email function that comes with mdadm. If a drive fails, I'll know right away and replace it with a spare. You do need your server to be able to send emails with something like postfix.
If you have hardware RAID, there's often a monitoring tool that comes with it or at the very least a command-line utility that can report the RAID state which you can then use in a script.
There's a protest coming up for this too (as well as to remember those killed and injured):
On Tuesday, October 22 join the active transportation community to remember those people using active transportation who have been killed or badly injured on Ottawa's roads in the past several weeks, and to protest Queen's Park's anti-cyclist proposed bike lane law. Meet on the north side of City Hall at 110 Laurier Avenue West at 5 pm for a rally and then protest ride.