How to clean an electric kettle?
You can use a solution of vinegar and water to descale your kettle (unless, as hobodave mentioned, the manufacturer advises against using vinegar). Fill your kettle with water until it's half full; add about 1/2 cup of vinegar to the water, and then allow it to boil for about 10 minutes. Allow the water to cool and unplug your kettle. Gently scrub away the scale with a non-metallic pad (don't scrub the heating element if it's exposed; wipe that with a sponge). Rinse your kettle several times after removing the scale, and then boil one or two fillings of water in it to ensure that you remove all traces of the vinegar.