KX - Moqups prototyping tool

As you will quickly learn through interacting with your client, using prototypes is probably the best way to communicate scope and vision to your client. Prototypes visually convey all of the complicated specs that you are trying to explain, and you will return to those prototypes many times to decide where to put buttons or what was supposed to be in the navigation menu. 

We found a wonderful tool called Moqups early on and it definitely shaped the way that we did our project. This is a free tool that allows you to design prototypes with common themes that you use every day. There is an IOS section that uses iPhone visuals so your prototype looks like the real deal. Theres another one for bootstrap so you can easily portray what the application would look like using your 333K skills. Overall, it was a wonderful tool and you should consider using it to make the most realistic prototypes at the least cost.

https://app.moqups.com/ (Links to an external site.)