ethtool
Queries network interfaces and can also set various parameters such as speed
netstat
Deprecated in favor of ss tool. Displays all active connections and routing tables; useful for monitoring performance and troubleshooting
nmap
Scan open ports on a network; important for security analysis
tcpdump
Dumps network traffic for analysis
iptraf
Monitors network traffic in text mode
mtr
Combines functionality of ping and traceroute and gives continuously updated display
dig
Tests DNS workingsl a good replacement for host and nslookup