Experimenting with creating web apps in the time it takes to write a blog post
chestergrant.posterous.com
chestergrant.posterous.com
Building a web app in 3 hours will leave you at best with something like threewords.me or its clones. While those can go viral, they rely heavily on luck. Why not spend 2 hours and build a beautiful and captivating launch page for an idea that would take you a weekend or two to build. Then, spend 1 hour reposting it everywhere and try to get feedback. This way, you can see if the concept itself gets any traction.
.. _: http://c2.com/cgi/wiki?IncrementalDelivery
Problems, however, arise when your idea requires specific domain knowledge which you haven't got yet; it's likely that you'll need some time to become comfortable in this particular area. Though I doubt it should necessarily be counted as time spent on app development.
And there's always some in-brain idea development. Say, you have an idea of blogging via email. You cook this idea inside for a few days, working out some details in background, then sit down, code for a three hours, and boom, you have posterous. Then you see that people don't care about email blogging, but they care about bells and whistles, so you sit another 3 hours and create basic skinning support. It's better than when after two weekends on email-blogging you discover that you were going in wrong direction. (Of course, I know nothing about real Posterous development, I just tried to clarify.)
Also, I think it's generally useful to be able to produce something in 3 hours. You need to keep your tools hot & ready.