This is absolutely one of the coolest applications out there. Balsamiq Mockups allows you to quickly and easily mockup any sort of desktop application, web application/website, or mobile application. Watch the demo and be amazed. For designers, this is incredibly powerful as you can make edits and changes as quickly—actually more so given the premade sketches—than with a computer layout tool, but your mockups will retain the loose, unfinished feel of a sketch, so clients won't feel like they are locked down into any one specific design. This thing is a must-have for anyone who regularly designs applications or websites.