There are lots of great mock up tools out there. Many have hand-drawn components ready for you to drag onto a wobbly scribbled-screen. In about 30 seconds you can mock up a screen with all of the usual controls that you would expect to find; check-boxes, inputs, buttons, drop-downs, links… and herein lies the problem.
When you reduce “design” to “dragging the typical things onto a typical screen” you become constrained by what you can see in the toolbox. To paraphrase one of the most over-used (but under-followed) pithy phrases about tools: When all you have are check-box, everything starts to look like a binary option. That little toolbar of pre-prepared controls is just waiting to blinker your design.
Mock-up tools seem like a great idea (great enough that you can now even get Microsoft Visio to operate in wobbly-line-mode) – but ultimately you are making the mistake of starting with the tool, not the problem.
As with most visualisations, you need to get back to basics. This means pen and paper, or a whiteboard, or some other form that offers few constraints and easy collaboration.
When I wrote this, I did a quick search to find a public domain example of a mock-up tool creation and a pencil-sketch creation. What did I find? Every time someone wanted to mock-up a list of “things” (products, pictures, music, video) – they all seemed to drag on a “cover-flow” mock up. How original. As you can tell, I closed my browser in disgust and abandoned finding an image.