Server Monitoring Solutions?

bwhaley asks: "The University I work for has asked me to research software solutions for server monitoring. More specifically, a piece of software that will monitor server variables such as load, swap usage, POP/IMAP processes, total processes, and all the other interesting data about a server's health. Watching these variables can give administrators advance warning about potential problems with the server. We are currently using an in-house solution written in Perl but its age is showing. I have found plenty of proprietary solutions such as HP OpenView and Sun Management Center, but these cost thousands of dollars. What solutions do Slashdot readers use? Are there any powerful open source solutions that I'm missing? Is anyone else running homegrown software that they are happy with? We are running an entirely Solaris environment but I am interested in any UNIX solution."