We've already developed a handful of mobile-ready HTML5 apps that are web-based, but store no data on our server, nor do we mine your data. Our newest addition was to scratch the itch of having to display a barcode at a gym/membership/club/store again, and carrying around this laminated plastic dongle that only annoyed me.
While the idea of showing a barcode on a mobile device sounds easy, it's not designed for this, and seems message-hostile (reducing a pithy amount of data to computer-read-only), but it works with today's high-resolution screens.
I was midway developing a system that would embed a font for display, and then considering graphic based solutions when I stumbled across this perfect javascript library that does all the heavy lifting with no external dependancies! I had to hack some of the CSS to get it to display large enough on mobile devices, and turned it into a one-page app that only displays the data fed to it via the address bar.