GNU Linux, clouds, applications development, web sites, servers, nginx, apache, tomcat, LEMP, MySQL/MariaDB, mongoDB, infrastructure setup, infrastructure monitoring, NGINX, Zabbix, Cacti, NewRelic, NetFlow, CISCO, Junniper, Huawei, PaloAlto, CheckPoint, Plesk, VestaCP, Zimbra, CentOS, FreeBSD, Debian, Ubuntu, Fedora, PHP, HTML, CSS, and several other tricks ;)