Dropbox and Github both use it to talk to their installed desktop clients from the website. It's very useful as it by just installing an app, a website can get more access to the local machine's hardware without having to resort to browser-specific plugins.

In our case, we're using it to read data from a barcode scanner to use it on a web application. All a user needs to do is to install the application an once that's done it works in all browsers.