The Unit Converter Widget is broken in Mac OS X 10.5.6: the left and right pop-up menus do not match, and Apple's code assumes they do. On the left menu, the Australian dollar appears at the top; on the right menu, it's the US Dollar. As a result, conversions come out incorrectly!
To fix this problem, make a copy of /Library/Widgets/Unit Converter.wdgt on your Desktop, Control-click on it and choose Show Package Contents from the pop-up menu, then open the UnitConverter.js file in a text editor. Search for "USD" (with the quotes). There should be only one matching line:
if (a.iso == "USD") return -1;
Delete this line. Then, delete the else at the start of the next line, i.e. change the line from this...
else if (a.name < b.name) return -1;
...to this...
if (a.name < b.name) return -1;
Save the changes and close the document. I suggest keeping the original widget in /Library/Widgets, and installing the corrected version in ~/Library/Widgets, so that the system's version remains untouched.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090225162441861