I just put up a page of some of the many prototypes I've designed over the years. Prototypes have been an essential part of my design process for years. I started as a broadcast designer utilizing storyboards as a way to test out, and pitch design ideas to clients. In broadcast design, and filmmaking, storyboards and animatics are the prototypes.
In interactive design prototypes are a great way to test out ideas without having to spend the time and money to built a fully functional site or applications. Prototypes can tell the story of an application, explore different interactions models--the different ways users could navigate an application, and allow designers to test out their ideas with users.
There are many different types of prototypes. They can tell stories:
Show an example of an application being used:
Or let a user interact and play with an interaction design:
How do you use prototypes in your design process?