Linux: Debian, CentOS
Virtualization: Xen – xend, XCP, xm/xl/libvirt/xapi; OpenVZ
Programming: UNIX shell, Python
Databases: MySQL, master-slave replication
Mail: sendmail, exim, postfix; IMAP; SPF, dnsbl, spamassassin
DNS: BIND
Web: Apache, nginx
Filesystem: DRBD, rados, ceph; rsync, backup, RAID, NFS
Misc: FTP, DHCP, Squid, ntp, Red5