Recently a web service called Joker has been in the technology press. It's a web application that downloads torrents based on magnet URIs, and allows users to stream video content and fast forward to any point in the video. Of course, it's already been taken down, and it didn't always work as well as my description sells it, but it was an interesting experiment all the same.
node-webkit-builder makes it easy to build cross-platform desktop apps with node-webkit:
Reading through these modules is like a showcase of Node's stream API. Academically they're fascinating, despite the obvious grey market connotations.
Which brings me to the TV/movie/music "PVR"-like applications. Media cataloguing doesn't have to be for pirated content: I have lots of DRM-free music, video, and books that could be presented in a better way. Combining my music purchases from Amazon, Apple, and Google into a cool desktop media browser powered by Node with a friendly RESTful API would be really fun and useful.
There's actually a node-webkit apps list, but I haven't yet found my perfect Node-powered media browser. Let me know if you've made your own Node media browser (or anything similar) and I'll check it out!