Well, those tools are really important, specially if your are in the design phase, currently I'm involving in the design, code and deployment of an application, the team members are not together all time, in fact, this is a project after work, so this is a great opportunity to test them.
Here it is a list of suggested tools, you may try them, I tried those ones:
1. iPhone Mockup, one of the fortress it's web based, so it's very easy to gather the ideas from all the team.
2. Pencil, it's platform free so I tried it on Linux, it's really a powerful tool, maybe if you are looking for something more solid, you may try it.
the rest of them are right here: http://goo.gl/sV9NWE
In fact, I got this list on google+ from this guy: https://plus.google.com/u/0/110885795581982251574/posts
let me know if you have any other suggestions or at least you try any other one from the list.
cheers
