the classic top util, used for real-time process list
top on steroids: color display, bar-graph per-CPU usage
apache-top is used to parse the server-status/ apache page (usage: apache-top.py http://servername/server-status, so the server needs - obviously - mod_status)
Note: Not to be confused with apachetop, which use apache's log files instead.
iftop monitors network usage.
MySQL real-time monitoring. access MySQL using the “mysqltop” user with no password by default, so create on the MySQL server.
iotop monitors, well, i/o.
pftop is a simple monitoring tool for pf, the packet filter for OpenBSD.
gotop top utility with beautiful text-graphs, written in go.
ctop monitors containers.
Powertop is a Linux tool to diagnose issues with power consumption and power management.