From Axure Software Solutions:
Axure RP gives you the wireframing, prototyping and specification tools needed to make informed design choices and persuade any skeptics. Quickly create beautiful wireframes with boxes, placeholders, shapes, and text. When you're ready for more visual polish, add color, gradients, and semi-transparent fills, import images, use grids and guides for precise placement, or the many other tools designed to help you work at the right fidelity for your project. You can change the sketchiness, switch to grayscale, and choose a handwriting font for your mockup instantly and at any stage of your project. Add notes to widgets and pages to provide more context to your wireframes and clarify or detail functionality.