PowerMockup

From Wulfsoft:

PowerMockup extends PowerPoint with a rich set of UI shapes for quickly sketching screens of web and desktop applications. With these shapes, you can easily create visuals of functional requirements instead of writing lengthy design documents no one wants to read. With just a few clicks, you can mock up complete user interfaces and iterate over them until all stakeholders are satisfied. As an add-in to PowerPoint, PowerMockup integrates perfectly with the Microsoft Office ecosystem, making it easy to embed screen mockups into other Office applications and simplifying the process of exchanging designs with colleagues and clients.

View larger image