A killer app is an app that makes a library or framework popular I think web.py is quite famous, But I do not know any large, widely used app written in the webpage.   Can you tell anyone? I've headline that the first version of youtube.com was coded using web.py, but I want you to mention an open source so that I can see its code. 
   
  Here the list of "real web apps" is written in web.py from the website.  
: A blogging and podcasting system  : A Flickr powered photo gallery   (v5): Play with colors A site for   todo: A simple webpage example where you can create, delete and edit-in-place  : A simple web app for music sharing (MP3 files).  : An iGoogle gadget directory written in MVC style.  : A very simple mass mailer.  : A system for rating, commenting, and accepting candidates for the conference and choice.  : Create randomly generated queries generated using Wikipedia.