From a Code for America blog post about Hyperarchy:
Hyperarchy is an online decision-making tool that allows team members to pose questions, submit answers, and rank the list in their preferred order. Under the hood, the system applies the Tideman method of ranked pairs to generate a consensus list from the sum of personal ratings. By imposing a logical structure on the messy process of collaboration, the system allows teams to deliberate without falling victim to groupthink or gridlock.
Hyperarchy is open-source and its source code is available on Github.
Update: Hyperarchy has been discontinued. In order to access the site, view an archived copy.