I have used a couple of different mockup tools throughout my career, such as Balsamiq Mockups and Moqups. I also know a bit of CSS and HTML, enough to modify existing webpages or create basic layouts. Furthermore, I also use Inkscape and GIMP to create basic icons, buttons and edit images, etc. In the end, depending on the needs, I am pretty capable of creating high or low fidelity mockups.