Operating systems:

Debian, Ubuntu, CentOS, Windows

Scripting:

Bash, VBS, CMD

Databases:

Microsoft SQL, MySQL, SQLite, Oracle

Web servers:

Apache, Nginx, Lighttpd

Virtualization:

VMware, ProxMox, Docker

Project & Process management:

Jira, Redmine, ServiceDesk

Version control system:

Git

Monitoring:

Zabbix, Nagios

Voice over IP:

Asterisk, FreePBX

Cloud computing:

AWS, GCP