Sequiturs
Sequiturs is a web app for developing rigorous, digestible arguments on any topic.
An argument is a series of premises and conclusions. Arguments can build on each other, by using the conclusion of one argument as a premise in another argument. Arguments can challenge each other, by marking the conclusion of one argument as contradicting a premise or conclusion in another argument.
On any argument, you can vote on whether each premise is true and whether the argument as a whole is valid, cite sources, leave comments, pose challenges, and revise the argument in light of the discussion.
Walkthrough
Sequiturs is no longer active at https://sequiturs.com.