HTML - the building blocks of websites. I have excellent knowledge of the current version, HTML5, and use it in all my projects to create truly modern sites.
CSS is perhaps the most powerful tool in my kit. Responsive sites and complex layouts can be achieved using CSS3, and with the rise of SASS it has become quicker and easier to write effective stylesheets.
Using PHP I can create basic content management for the owners of the sites I build, and add even more functionality to my sites such as sending messages from forms or dynamically generating content.
With MySQL and PHP, I can add guestbooks and simple blogs to websites, and create catalogues of products for online stores.
Wordpress is very useful for larger sites or more complex blogs, and allows site owners to easily manage their content. I often use my own custom theme to build the site, and sometimes create child themes from existing templates.