I am working in a couple of websites and I needed a new mockup tool, usually I do my mockups in Visio but I really wanted to find something simpler to work with, a software that would allow me to create my mockups in less time without compromising quality. I did a Google search and found out about this great software called Balsamiq Mockups. Balsamiq is pretty easy to use, the interface is great and it comes with 75 pre-built controls which help you design anything from a simple interface to a complex website or Rich Internet Application. If you usually create your designs by hand you'll love this software, all UI elements were created by hand, this is so your clients don't get attached to things such as “that pretty color gradient” or think that your mockup has actual code behind it and is “practically done” as Balsamiq describes it in their website.
After I installed this software (the Desktop version) in my Mac laptop I was able to create a preliminary design for the website I am currently working on in about 10 minutes. Balsamiq designs can be exported, saved, etc. You can run Balsamiq on Windows, Mac or Linux.
I really like this tool, it is helping us be more productive and th designs look great. If you have sometime I recommend you visit Balsamiq's website and take a look by yourself at this software.
Balsamiq Mockups website
Sample Mockups