I have been doing this for a while. These are the main ones. There are many that I haven’t listed. Ask, if you need an expertise you don’t see.

PCI Compliance

SSL certificate purchase and installation (for HTTPS)

Proxy servers for DDOS attacks, caching, optimizing.

Databases (MySQL, MariaDB, Oracle, MSSQL, Paradox, MS Access, and lots more.)

PHP, CSS, Javascript, LESS, Python, Perl

LAMP server administration (LINUX, Apache, MySQL, PHP)

CENTOS, UBUNTU server set-up specialist

Command line administration of most LINUX servers

WordPress, Drupal 7 & 8, Magento 1.9 and 2.x, CMS systems in general

Moodle

Gmail administration, WHM, CPANEL, Git, VS