A fair amount of software products – I would dare say about 70% – have front ends (or User Interface, UI, if you will). Narrow that list to application products and the proportion possibly goes up to about one hundred percent. The final output of the product development process is also a great artifact to start off the initiative. Here is how it moves
The above diagram should be self-explanatory. If your product relies on a visual impact and superior usability as core differentiators then please do not leave that job to your developers. Visual Engineers and Usability teams usually create two important artifacts – a visual representation (from 2) and a fully or quasi-clickable html mock-up (from 4). Do not throw these away. Use the HTML mock up to create an early adapter program (pre-alpha) for your product and create a feedback loop back into the Usability team. Show the clickable model to senior sales folks to excite them about what is coming up as a salable product down the road. Plug the Photoshop based static output into presentation and other non-interactive product collaterals.
Start with the finished product. It is a fantastic place to begin.