I have recently found a great online gallery of HTML5 web app designs. It has 19 (at the time of this post writing) well designed and free to use web app templates.
They are all responsive, look beautiful on desktop and mobile browsers and do support multiple screen resolutions.
What I most like about this web gallery is that the author has started in 2012 and has grown a lot with every new design he posted! He has also created a www front end framework skelJS which is the powering engine for all these HTML5 designs.
The most recent templates are really worth taking a look at and will for sure give you some inspirations on how to use HTML5 in web projects.