The acidity in lemons are really good for removing limescale. If you stick them on the nozzle of a tap, it helps get to the hard to reach places. For the shower, we use a preventative spray cleaner to stop the limescale building up in the first place, while with the kettle, a gust of boiling water can start the removal process.
We use Enjo at home most of the time. It's just the cloth and water, but it seems to do the trick. They were really popular for a while here in Australia, but you don't hear about them so much any more.