The vector shapes and overall style are reminiscent of a hand-drawn feel and it sets you in the creative mood. Buttons and drop downs are very accessible and organized as drag and drops go.

nijamock support Android, iPhone, ipad, Windows Phone, surface, web page etc.

Ninja Mock it's free for non commercial use. I think this is the first and most important point. You have unlimited number of users, and the interface is not so difficult, so basically you can easily kick off the learning curve. Good to see that the platform integrates itself on many devices, so it's easy to share our work with team and clients within Ninja Mock.