I have been asked several times already which UI prototyping tools I know or which ones I could recommend.

Typically, there is no all-in-one tool suitable for all prototyping purposes. But I decided to summarize some tools I already applied here in my blog.

• Justinmind (http://www.justinmind.com): From wireframes to simulations. Acceptable price for single user license
  
• Axure Pro (http://www.axure.com): very advanced tool, great support for developing detailed protoypes, but strong web focus. Additional widget libraries (e.g. http://www.acleandesign.com/2008/11/the-top-10-axure-resources/, or http://www.acleandesign.com/2008/09/axure-design-pattern-library-v2/) help to build quite interactive UI prototypes, which can be employed as UI specification. Belongs to my tools of the daily business.
  
• Mockupscreens (http://mockupscreens.com): very cheap tool for building static UI mockups
• Microsoft Visio: if your company own Visio licenses, this might be a rational choice. However, the UI prototyping support is rather poor and heavily relies on plug ins such as the one from http://www.guuui.com/issues/02_03_02.php
• Adobe Photopshop, Dreamweaver, Flash, Flex (http://www.adobe.com): I used tools of the Adobe suite when things have to look pretty (Photoshop) or the prototype needs to have special interactive behavior (Flash, Flex). The latter demand some ActionScript know how.
• Microsoft Expression Blend (http://www.microsoft.com/expression/products/Overview.aspx?key=blend): the new star on the horizon for usability professionals. With Blend 3, Microsoft will enter the market with specific prototyping features built into a tool that can be used along the whole UI development supply chain. A great playground for usability people and UI designers, who have some C# skills.
  
• iRise Studio (http://www.iRise.com): one of the most sophisticated prototyping tools, the big brother of Axure Pro. Very expensive, but very advanced. For more interactive UIs, Flash movies can be plugged-in.
  

I have also heard (but never used) the following tools:

• RapidRabbit (https://rapidrabb.it/de): very motivated approach, I expect to see more mature versions in the future. We need more tools like this!
• OmniGraffle (MAC only)
• Serena Composer (http://www.serena.com/geo/de/products/composer/)
  

Another very good overview on prototyping tools can be found at
http://www.adaptivepath.com/blog/2009/03/24/rapid-prototyping-tools/