http://h34nug3iwqjf2376fmmzdhgqivckzemcujmarlptfqeqtq7vya47yyad.onion/2007/09/page/2/index.html
Among other things, I monitor disk space, memory usage, system load, network download and upload speed, internet connections by protocol, RAM usage, swap usage and running processes. Although it takes up a bit more memory to do so, I also monitor the /var/log/messages file (the same as having a window open running tail -f /var/log/messages just to see if anything is happening behind the scenes that I should be aware of.