I've seen this one in action and it works really well. It's not a touchscreen like those on your phone, the screen edges work like a camera to find where your fingers and where the pens are. It works like a normal screen attached to your PC and then there's a Windows application that lets you make annotations beyond what the normal OCR and OneNote drawings allow. I'm trying to get my company to buy at least one of these.
They also have smart whiteboards that scan their content and broadcast it to mobile phone apps. It's not bi-directional interactive like the big Smart screen but it's only £500 I think.