Another free alternative:
https://moqups.com

This one is built using HTML5 and at this point doesn't seem to include colaboration support.

Recently I've found some apps from ZURBapps, they look really cool:

https://verifyapp.com/
usability/concept testing tool - has several available tests
it's paid, but has a 30-day free trial period

https://www.influenceapp.com/
great tool for graphic designers to ask for feedback and comments - it's in beta tests, so it's available for free at the moment, you just need to request invitation (I waited a few days)

https://www.solidifyapp.com/
another tool in beta version, thus free - this one for creating clickable prototypes

Hi,

Here's another interesting alternative and free alternativae. Maqetta is a browser based open-source tool for wireframing.
Check it out at:

https://maqetta.org

Hi guys,

I'd like to invite you to reshare the links in this thread:
https://www.outsystems.com/NetworkForums/ViewTopic.aspx?TopicId=9657&Topic=Mockup-Builder-Free-Application

Actually, most of the content we've seen on this Usability Internal* Forum could be made public (I'd like to hear your opinions on this).

*why internal?

Cheers,
Davide

My 2 cents: I think it should be made public. There's no info here that could not be made public and would certanly help someone.

Yes, please! Let's unlock the MAGIC!

But I think Davide's idea is better - re-share there so that Gonçalo Cardoso (the guy that started the other thread) gets a cool reward for starting it. 

Natalia is already on it! :)

Regarding the other threads we could repost* them gradually and create an [hopefully] noticiable trend towards usability in the months leading up to NextStep.

*just moving the threads across forums would seem strange (and we'd have to validate what we wrote to ensure that it could be made public)