Without a doubt, one of the most challenging web applications I helped built. This project was part of a two man development team, with me taking the front-engineering work (an extensive amount of AJAX) and the other developer taking over back-end work.
A two month development cycle, followed by a one month QA and testing led to this project being incredibably succesful as we were able to fine-tune the application and making sure it was cross browser compatiable.
The application was engineered well enough to be featured on TechCrunch on the day of launch: http://www.techcrunch.com/2008/11/11/dukaus-launches-free-conference-calling-realizes-it-wont-make-money/