Rather than assemble content in server-side processes, static web applications rely on the user's browser to drive interaction and content rendering. Once considered only useful for static content, modern static web apps can dynamically fetch data, synchronize multiple users in real time, and more.
While I think web developers who want to build apps should seek fluency in a server-side technology, there is a place for static web sites for small micro apps and MVP prototypes. Thanks to the rise of APIs that support CORS and services like Parse that provide a ready-to-go backend, these apps are more powerful than ever.
GitHub is also sponsoring an upcoming hackathon, the first such contest I've seen just for static web apps.