processes

(h)top
strace

i/o

iotop
(lm_)sensors

network

iftop
netstat
nethogs
jnettop
ss
route
host
dig
traceroute
iwconfig
iptraf
wireshark
iperf
tcpdump

hardware

lshw
smartctl
snmp

hdd/ssd

ledctl [ledmon]
find /sys -name locate
echo <01> > <file>

power

acpi
upower

infrastructure

ganglia
icinga
nagios
shinken
xymon
zabbix

http calls

httpie
curl

json

httpie
jq
jo
python -m json.tool
json_reformat

yaml

yq