I've got a couple of guide rails for my Makita circular saw that do the same thing. They've been extremely useful for ripping long lengths of timber, like the bamboo flooring I laid in around 2/3rd of the new house. Here's a link to the Makita guide rails: http://www.makitauk.com/accessories/cutting/guide-rails.html
You could probably make something for less money that would do the job, as in that photo, but the big advantage of the custom extrusions, like the Makita ones, is that they are light and very easy to position and use. The 1.4m rail was around £50, IIRC, not sure how much the 3m rail is, as I bought mine second hand.