On the basic model B you could hook it up and load up an operating system and you're away. I can't remember which O/S we went for in the end but it was one that included "scratch" in the bundle as we're teaching our boys to code. Ours are a bit young for anything else, scratch is quite good fun for them. We have one with a few addons, the addons we have involved soldering connectors to the main board and then simple wires out to a bread board to get it all hooked up. So the boys have added connectors to output to a little LCD Display which is controlled from the PI along with some distance detectors. So far they've got the distance detector to output the distance it's measuring to the LCD to make a height measurer. We're looking at getting some lego weDo for the boys for Christmas which can be controlled by scratch, and then a buggy type thing with the distance detectors so it can detect obstacles and turn, that's a long way off though! If you want to know more specific details I'll ask my hubby, he uses it as a boys project, I just know how it roughly works as I'm into that kind of thing! It took months to arrive and when it did it wouldn't work. We got as far getting Farnell to say they would ring to arrange sending a courier, but they didn't - must get things moving