What are some free tools for collaborative UI prototyping?

Question

Layouts and prototypes

There is Ninjamock, it's cool, but it can't be used for collaborative prototyping. You can show the prototype to your friends, but you can't do it together.
Are there any free collaborative tools for creating website layouts?
Ninjamock + Google Docs - something like that.

Answer 1

invision app?

Answer 2

Google Docs + draw.io = ставите плагин в гугл-докс и все отлично работает.