Pet Project: Sync Subs
I have been working recently on Sync Subs. It is a website that allows you to synchronize a file with subtitles (in SRT format) applying a time offset: 
There are lots of websites doing this by letting a user upload a file and then applying the transformation server-side. But with the HTML5 File API you can do this client-side. Thus, you can read the contents of a file, process it and let the user download a file that has been dynamically generated using Javascript in the browser.