TeuxDeux is a Web and iPhone based task management application produced by a collaboration between Swissmiss and Fictive Kin.
According to PC World the visual layout of the application facilitates the Getting Things Done system of task management.
[1] The application is considered notably useful by Lifehacker[2] and Gizmodo.
[3] According to the developer's website, the backend of the app is written in Ruby, with Sinatra serving pages and Grape delivering the API.
The front end is built on Spine.js.