I highly recommend Nagios for system monitoring. Our setup monitors a variety of parameters (including free disk space) on about forty servers, as well as the numerous installations of the web application that I wrote and currently support for a variety of clients.
Some problems just send me E-Mail, and other problems page my Blackberry 24/7.
Way better than anything I could written myself, and very flexible.
Alex / talexb / Toronto
"Groklaw is the open-source mentality applied to legal research" ~ Linus Torvalds