ConvertIt is the premier unit conversion program for the Mac OS X platform. With 2800 units, including an up-to-date currency section, it features a scientific calculator, tabbed windows, unit highlighting, online definitions and much, much more.
ConvertIt 3 allows for having multiple windows open at once so you can go back and forth between categories or even within the same category without losing the conversions you just did. You can have as many tabs as your heart desires.