Single page, application websites (or microsites) are often used to replicate the experience of native apps, especially mobile apps. Such applications are designed to act like native apps by rarely or never needing to be refreshed. They often are developed using frontend javascript to create dynamic user interactivity. For a long-time, such a reliance on javascript was discouraged on the web for a number of reasons, most notably since it can break the "back" button on browsers and it's inability to deep link into the application or website.
Such website applications are becoming more common. Twitter and Facebook are examples of such applications and are testament to their growing popularity. Web designers are adopting the #! / web app mentality ever more, as the approach is spawning new, rich forms of interaction online.
HTML5 will help single, page web apps become more exciting, engaging and usable. Problems, such as breaking the browser "back" button, will be relieved by the new capabilities of HTML5 (HTML5 History API will help solve this issue for example). The promise of HTML5 has already helped to dethrone Flash, as a staple for interactive web design and development. Adobe has ceased to support Flash for mobile at a time when mobile is becoming more of a necessity than a luxury for businesses, brands and organizations.
The new capabilities of HTML5 will power a new breed of websites, applications and microsites and create a new level of interactivity online that will be user-experience and data centric.
Single, page web apps have already become a steadfast part of mobile web development. With the broad support for HTML5 on smartphones, native apps are nolonger required universally, although at times they may be the best tool for the job. In many instances, website applications liberate brands from entrenched mobile marketplaces and still provide excellent functionality and marketability. In a world where the "cloud" buzzword breeds excitement and possibility, the web is finding new ways of achieving accessibility and powering rich interactivity. Single, page web apps will have their place in this next phase of web development.