Monotask was an attention management application, a joint startup effort between Charlie Park and me. We worked on it from 2011 to 2013.
It consisted of a Rails application that would allow users to specify a weekly schedule of sites to block, to help our users stay focused on their work. There was also a client application, written in C++/Qt, that would synchronize with the web application and use the schedule data to control an HTTP proxy server (a custom Apache build). The proxy server would disallow access to the desired sites using rewrite rules.
The client software was my primary focus; it went through several iterations. We were initially targeting Windows and Mac, and the first version was a C#/.NET/Mono application. At the time there was poor support for certain OS X features, so I moved to a C++/Qt client that worked quite well. Parts of the application needed OS-specific C++ and C code, and the platform was flexible enough to allow this.
The biggest pain point was reliably controlling the Apache instance; it would sometimes not come back up after hibernation, for example, and was difficult to restart on Windows. At the time that we closed the project down, I was investigating various ways to integrate the proxy more fully into the control application; I was looking at a Node.js solution for this, and also experimenting with my own C++ HTTP forward proxy.
We decided to shut the project down because we weren't able to meet our subscriber goals, but we were both very satisfied with the application we'd built.