Duo is a browser for responsive web developers that allows you to simultaneously build sites in two different sized frames.
Electric Pulp had the idea to build a web browser that would allow you to view and build sites simultaneously in both a mobile size and a desktop size. I thought it was a fantastic idea. Since I had been working on Blind Browser, I was familiar with some of the quirks of building a browser for OS X. I whipped up a prototype one afternoon and it was immediately clear that this was a useful tool that needed to be built for real.
Building the app was not easy. It was important that the two frames always took the same actions, but were also rendered independently. Performance was the biggest issue. The communication between the two frames was the majority of the work.
We added WebKit Dev Tools to allow Duo to be useful as your primary development browser. We replicated all the basic Safari key commands so it felt like a full fledged browser as well.
Finally we added in the URL scheme bookmarklet technology of Blind to allow users to send sites to Duo from their current browser. I find this to be the best feature of Duo.
Since Duo was released, I no longer am a part of Electric Pulp, but they have continued to develop Duo. The most recent update includes a resizable left frame and a customizable toolbar.
Check it out on the App Store.