This calculator tells you if you are overweight or underweight, as per the Body Mass Index. Remember though - this doesn't take into account the size of your frame, so take the results with a pinch of salt.
Take a look at this chart to see how your weight is classified. Again this is very approximate and ignores many factors, so please don't take it too seriously. It is a useful reality check though, if you think you may be obese or you are under-eating.
Here are some more tools you might find useful.