A very simple web app built for a friend of mine who is a cycling enthusiast. If you move from one bike to another, you want the relative positions of the seat and the handlebars to be the same, because that's the position you're used to riding in -- but the sizes of the two bikes are different. So, how do you adjust the various parts of the new bike so that the handlebars are in the same place as on your old bike relative to the seat? That's what this app calculates, or at least lets you experiment with visually. If it had a big catalogue of bike dimensions for all sorts of different bikes, I think it'd be a useful little tool for cycling enthusiasts.