What Tools Do You Use for UI Prototyping?

AccUser asks: "There are many articles discussing methods of UI prototyping. Having been involved in the design and implementation of a number of commercial applications (both desktop and embedded), I know the value of producing early prototypes of the UI. In the past I have used visual programming tools, such as Visual Basic, but there is always that request: 'Can't you just complete the prototype and release it?' One project I was involved with, the UI prototype employed hand drawn graphics (including hand written text labels, etc) in order to be explicit about the fact that it was a prototype. What I would like to know is what tools and techniques do you use for UI prototyping, and how do you manage your client's expectations?"