First of all, thanks to fellow OU student William for bringing this to my attention. Balsamiq is a web app that allows you to mock up wireframe user interfaces through a web application or desktop application.
The application can be used for free through the web interface but it will prompt you every 5 minutes to say thank you for using it, otherwise you can get a one month free trial of the web tool followed by a $12 (£8) per month subscription or $79(£50) one off fee for the desktop application. To be honest the free web access with prompts will be more than good enough for my needs, I'd highly recommend it to anyone!
Visit the url here and try it out http://www.balsamiq.com/