We have extremely hard water here (meaning that it has a high mineral content). It really does a number on my tea kettle. When I looked in it this morning, I couldn't believe how disgusting it looked!
So I put about half a cup of vinegar in it, and filled it up with water. I boiled this mixture for about 10 minutes (making the house smell like a pickle plant) and it looks SO much better! You can also run vinegar through your coffee maker when the minerals start to build up on it. Of course run plain water through afterward so your coffee won't taste bad the next morning. :)