We were busy yesterday looking into some performance issues with our mail server - I was lucky enough to learn some new Linux commands (these all work on Ubuntu Intrepid, however iotop does not work on RHEL 5.2). iotop - allows you to see which processes are performing the most reads and writes to disk.
iptraf - allows you to see (in real time) who is talking to your box (sort of a lightweight ncurses version of ethereal).
dstat - combines iostat, vmstat and ifstat into a sweet package that gives you a good idea of cpu, disk io and network traffic volumes on your hosts (colour coded so you can pick up trends).
