This is not precisely UX related, but very important to user interface design in general. In summary, across applications (Photoshop, Dreamweaver, Flash) and platforms (Browsers, .NET, Mac, Silverlight, etc) font sizes are always inconsistent. Some measure it in points, pixels, em, mm, twips and others, but 20 points in Photoshop is always different from 20 points in Dreamweaver.
This makes it very hard to judge font sizes for GUI and web page user interface design in general, and its impossible to memorize numbers since the same number means different things to different apps and platforms.
So is there a unified metric or calculator that gives you corresponding font sizes across applications?