Here is a sketch of one made with bCisive online, the best of what I have evaluated so far. If you sign up it looks as if I can invite you in to edit collaboratively.

It's easy to use and I find the visualization useful, but it bothers me that it's basically just a mind map, you can't add semantic information about how plausible you find various arguments. Another app with much the same characteristics is Debategraph.

I have tried ArguNet, which is supposed to reconstruct logical structure, but the UI is unusable.