For years I have been using the gear calc on sheldonbrown, similar results to above. But recently, someone posted about:

gear-calculator.com

And I must say, it offers the additional advantage of a visual representation of the spacing, which is also "normalized" to (I think) a logarithimic scale of "development", meaning, for example, 1 tooth between 11/12 does not equate to the same visual representation as one tooth 20/21.