POP by Marvel: POP helps you transform your pen and paper ideas into an interactive iPhone or Android prototype.

Noun Project: Free icons for everything.

Unsplash: Free high-resolution photos.

Sketch: Sketch is a wireframing app that was created to design interfaces, websites, icons, etc.

Invision: A prototyping platform for team collaboration and user feedback gathering. For free student access, use your NYU email and this security code: 56-73-13-19.

Balsamiq: Balsamiq Mockups is a rapid wireframing tool that helps you work faster and smarter.