Frame Box
http://framebox.org/

Frame Box is completely free to use which may make it the right stop for those teams on incredibly limited budgets. The downside is that it’s a bit basic and perhaps not the best tool for developing a detailed wireframe of a complex project on. It’s perfectly OK for smaller, less complex projects though. They think it’s unlikely that Frame Box is going to be anyone’s permanent option but it will get you started and without any strain on your wallet. This will be added to the tools section of Research Resources Subject Tracer™.