Linux system administration, networking, security, performance tuning, scripting.
High availability, load balancers, failover, GEOIP setups.
Server storage / distributed storage: RAID, SAN, LVS, iSCSI, DRBD, GFS2, glusterfs.
Shell/bash, awk, sed, perl scripts.
PHP, Apache (including WHM/cPanel and Plesk setups), DNS, SSH, etc. - installation and configuration.
Mail (SMTP/IMAP/POP3, anti-spam, anti-virus solutions); solving mail deliverability issues.
Database tuning / troubleshooting: PostgreSQL and MySQL.
Virtualization - Xen