User Interface Design Tool

I'm searching for a User Interface Design tool to visualize a possible GUI in a documentation. I must not generate code. I know that Microsoft Visio provides a functionality. But are there any alternatives?

Which software do you use for visualizing a GUI?

Thank you in advance.
Best regards from Switzerland
Thomas

PS. Another well-known tool is The Pencil Project.

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Thank you for all your replies!!
I'm very fascinated from Balsamiq Mockups! But finally I'm going to use The Pencil Project.

From stackoverflow

• An awesome and easy one is balsamiq: http://www.balsamiq.com/

  I also quite like Axure.

• The "must not" seems odd... if you use an IDE designer to throw some controls down, but don't use the code, is that sufficient?

  Or use "blend" - that generates xaml, which is just layout - no code.

• If you are in Linux you might try Glade.

• Give a look to Firefox Pencil Extension, is a basic tool for GUI prototyping and simple sketching....

• Take a look at the tools mentioned in the answers to this question:

  http://stackoverflow.com/questions/517324/prototyping-tools-for-non-programmers-expression

  I'd still recommend Balsamiq Mockups. The resulting GUI mockups look like sketches, making it a great tool for communicating the idea that this is not the final GUI, but merely a visualization of GUI concepts.

• You may want to have a look at Joel's article The Iceberg Secret, Revealed, as well as the articles discussion.

  I don't know wether this is relevant to yout task, but since reading it I definitely prefer to use handdrawns sketches of a UI if I want to present it to a customer. (A 'Pencil Project' in the true sense of the meaning ;-)