Collection of tools for analyzing server security, network and access statistics:

iftop - a live top style view of network interface traffic, a more user friendly tcpdump
htop - a live top replacement for system process, memory and disk usage in one view
nload - another live view of network interface traffic
vnStat - provides continuous, historic network interface statistics. Command line or html output.
nethogs - which IPs are using data currently
iptraf - ncurses based network traffic analysis
ncdu - term based large file analysis

Web tools
vnstati - used with vnStat for graphed image output
darkstat - provides live and historic traffic graphs and src/dest port and IP information
fail2ban-dashboard - provides graphical reporting on fail2ban jails with a local server
GoAccess - analyzes your apache2 logs and produces an impressive web analysis of popular stats
Logwatch - analyzes syslog and reports on major issues and fiundings by host and process; produces emails, command line or html output
Webalizer - old school apache2 log analysis and basic graphs, produces html
pflogsumm - text analysis summary of postfix logs
rspamd - has a web interface to monitor performance of anti-spam system rspamd

.bashrc
alias h='history'
alias mess='tail -f -n 50 /var/log/syslog | ccze -A'
alias htlog='tail -f -n 50 /var/log/apache2/access.log | ccze -A'
alias mmess='tail -f -n 50 /var/log/mail.log | ccze -A'
alias spam='/usr/bin/rspamc stat; tail /var/log/rspamd/rspamd.log | ccze -A'
alias fail="/usr/bin/fail2ban-client banned | tr ']', '\n' | tr ':', '\n' | column"
alias update='apt-get update; apt-get upgrade'
alias bigfile='cd /; find . -type f -size +3M -printf "%s %t %p\n" > /tmp/tbigfile; egrep -v proc /tmp/tbigfile | sort -g; rm /tmp/tbigfile'
alias mailfrom="grep connect /var/log/syslog | awk '{print $6}' | sort | uniq -c | sort -nr | more"

Other
column - pipe commands through column to make efficient visual use of terminal
ccze - pipe command for nice log coloring
ncdu - terminal ncurses big file finder
speedtest-cli - terminal based speedtest to Internet

These tools can be installed with common package managers or from source. For recurring page or log analysis, add to cron.