A killer app is an app that make a library or framework famous. I think web.py is quite famous, but I don't know any big, widely used app written in web.py.
Could you point out any? I've head that the first version of youtube.com was coded using web.py but I'd like you to mention an open source one so I can see its code.
Well, when Reddit first moved from Common Lisp to Python they used web.py (src, src). That made a pretty big splash at the time, which might explain some of its popularity. It should be said, I guess, that Reddit has since abandoned web.py in favor of Pylons.
From web.py website here is a list of "Real Web Apps" written in web.py. None of them has yet become the next twitter.
A killer app isn't necessarily defined by whether it makes the framework/language famous. The Killer 'App' is an application that disrupts its industry in such a way that the rules or status quo of that industry is changed forever. E.g. the iPhone.
It is 'killer' because it 'kills' off market leaders in its industry.
A by-product of a killer app is that the technology used to bring it to market gets a fair amount of media attention. This is inevitably happens because the majority of developers(in our case) who operate in the killer app's industry want to be up on the latest trends/concepts/etc...
In the iPhone's case: It has sold millions; meaning the iPhone is a cash cow; therefore developers flock; developers need to then know what technology to use; more developers=more attention; etc...
Obviously this is a short summary on what generally happens.
Anyway... Pylons is pretty good. Its gaining quite a bit of attention because it is easily customizable, robust and focused on doing what is necessary which is rapid development.
There is also Turbo Gears, but the latest version is built on Pylons anyway. Hope this helps.