OS - Linux, FreeBSD

Routing protocol - BGP, OSPF

Mail service - exim, postfix, sendmail, courier,dovecot (dkim, spf, dmark, spamassassin, clamav...)

DNS - bind, nsd, unbound

Web servers - apache, nginx, lighthttpd

DB - MySQL, Postgresql, mongodb

Virtualization - KVM, Xen, docker

Monitoring and visualization system - cacti, mrtg,zabbix,nagios, netdata, graphite (grafana), snmptt

VoIP - asterisk (GSM, FXO-FXS gateway, sip,iax2- trunks)

VPN - L2TP, PPTP,IPSec,PPPoE,OpenVPN (Cisco, Mikrotik, Linux) + Freeradius

Scripting languages - Bash, Perl, PHP