To watch a number of system parameters and the overall performance of my homeserver (and my workstation when its powered on), I run munin.
Its a nice tool in general and it provides a nice history of events. It can even send you emails and inform nagios when something happens. Both features I haven't tested yet due to a lack of mta and nagios...
But some things you should be aware of:
When you want the disks to spin down when they aren't needed, you should not run any munin-tool that uses smartctl. Despite the fact that smartctl knows the parameter --nocheck=standby, that doesn't keep smartctl from bringing the disks online.
- The smartctl-output is only checked from 5pm to 1am when the disks are generally not in powersave-mode.
- To watch their temperature I use the munin-tool using hddtemp which doesn't wake up the disks.