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.

Debian-admin article

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.