I'm surprised no-one has mentioned
Citronensäure (citric acid) yet. I used to swear by Surig Essig Essenz, until I discovered this:
Heitmann Citronensäure (scroll down the page a bit)

This stuff is a good bit more powerful than Essigessenz for removing limescale, and smells faintly of nice fresh lemons, much more pleasant than vinegar. You could in theory even make lemonade with it if you watered it down enough. They have it at Müller Drogeriemarkt, don't know about other places. I've only used the liquid version in the bottle so far, haven't yet tried the powder version. But I love it!

Please please everyone, go out and buy it, so the manufacturer stays in business!
Oh, and I second BadDoggie's advice about not inhaling the fumes if you use vinegar + heat. That can be nasty!