- top
- htop
- vmstat (system activity, hardware, and system info)
- w (who is logged in and what are they doing)
- fuser (what processes use the named files, sockets, or filesystems)
- uptime
- ps
- free (memory usage)
- iostat (average CPU load, disk activity)
- sar (collect and report system activity)
- mpstat (multiprocessor usage)
- pmap (process memory usage)
- netstat (network statistics)
- ss (dump socket statistics)
- iptraf (real-time network statistics)
- tcpdump
- strace (system calls)
- /proc filesystem (cat /proc/cpuinfo, for example)
- nmap (port scanner)
- lsof (list open files, network connections, etc.)
- ntop (web-based top utility)
- vnstat (console-based network traffic monitor)
- mtr (badass traceroute)
- dstat (combination of vmstat, iostat, ifstat, netstat, etc.)
- dtrace
- whowatch (realtime login/connection info)
- atop
- iftop
- iotop
- jnettop
- LTTng
- ngrep