I have been using a tool that is great for mocking up web sites and for icon creation. The app is called Sketch and it is aimed toward the web and device u/i designers.
The latest update allows for prototyping a web site or mobile app that can have some limited functionality for navigation between pages and other actions. The app also has tools for flexible/responsive apps and web pages. This gives the designer the ability to show how well the web site looks on any size screen. And for device apps it will give the client or any other decision makers how the web page looks or how the iOS or Android app looks on a particular device.
The advantage of using an app like this is that allows a designer to show what a potential product will look like before the app or web site is committed to code. The client or Project Manager can make design changes and suggestions which saves on production cost prior to the more difficult phase of actually coding the product.
I have used it to make the logo for this site and I have just started to use it for web page mock up for my personal site as well. I am very excited about this tool for designing web sites and other graphics that I create.