Pidoco (www.pidoco.com) is a great tool for mobile apps and collaboration at the same time. It's online based, so you can share your prototype and simultaneous

Pidoco (www.pidoco.com) is a great tool for mobile apps and collaboration at the same time. It's online based, so you can share your prototype and simultaneously work on it and with the Pidoco app, you can preview your prototypes on your mobile device. Also, it is very intuitive and doesn't require any technical knowledge but still offers a bunch of different interactions and stuff. For me Pidoco is working best for the wireframing and low fidelity prototyping stage.