I don't think that they are as accurate as we would like them to be... the biggest problem is all of the variables. Depending on your personal weight and height and body make-up we will all burn calories a bit differently.
One good source that I have found is You insert your personal information and then select the exercise.. it also has a really good overview about what your resting metabolic rate and all of those good things are.
However, the problem you will run into is that not all treadmills are alike, and not all stairclimbers are alike, etc.
My best advice... good luck, do the best you can, and know that there is such a thing as grace in the world... if you are working out and eating healthy, you can probably expect good things - as long as you are in the ballpark for numbers...