HTML5 Up
HTML5 Up makes spiffy HTML5 site templates that are responsive, built on HTML5 + CSS3, super customizable and 100% free under the Creative Commons license.
Related Tools
Webflow Templates
Free website templates for designers, businesses, and personal use. You can browse free business, portfolio, and blog HTML5 responsive website templates for Webflow.
Colorlib
Colorlib offers some of the best free WordPress themes, responsive and full of options, ranging from business themes to blogs and portfolios.
Share Bootstrap
Share bootstrap offers great free themes and templates built with Bootstrap. It is an especially great resource for Dashboard templates and themes.
One Page Love Templates
A collection of 80+ free one page website templates. These free templates are great to quickly test your ideas before committing to a big project development.