A free chapter from Todd Warfel's book - Prototyping: A Practitioner's Guide.
Todd's research found website developers and usability professionals using a wide range of tools and techniques. Some of them pretty surprising. Excel anyone?
The bottom line here is use whatever you're comfortable with. Whatever will get you what you need as quickly and as easily as possible.
Observing the participant's interaction and talking about it is what's important, not the tool used to create the prototype, after all.
Number one on the list is paper and pencil. No learning curve with that one.
Prototyping: Picking the right tool - by Todd Zaki Warfel
I blogged Todd's book a little while ago, with a link to another free chapter. His book is now on my shelf.
Prototyping book by Todd Warfel