linuxNagios
I've now got a full Nagios build running at home, monitoring various systems (my ADSL router, the bare metal Xen host, all the guests, my NAS etc) in my house. I originally installed it simply so I could check when updates were available for the Debian systems I have running here (rather than having to apt-get update manually to check) by using snmp coupled with a script that checks every night for updates but soon got carried away and now I'm monitoring and graphing (with cacti) a whole bunch of stuff.
I've installed Nagios before but it was a long time ago - either I knew less back then than I do now (quite possibly!) or it's got a lot easier because within about an hour I had monitoring, email alerts etc all working (and that time includes building the virtual host!)
<3 Nagios