After a bit of tweaking, I have setup the £40 Wiimote touchscreen, next, setup the projector at school to be a free Interactive whiteboard. Here is a video of me using the laptop screen as a touchscreen. It's a phone video so poor quality. Just a wiimote £20, and Infra red led pen, £5 and a USB bluetooth dongle, £7. Wiimote freeboard freeware software and MSNet3, a free download.
Twisting the projector to point straight down onto a desk then having 4 pupils all sketching/designing/modelling/interacting with the PC and each other, at the same time.
All the software and info here.