There are now two versions of the converter, new (Swing) and old (AWT). I recommend trying the newer one, and if it fails you can fall back to the older one. You can probably run either in your browser, but the newer one also runs directly as an application, and seems to work better that way.
Rather than requiring lots of data, the algorithm knows the basic conversions within each system (e.g. 12 inches in a foot) and then uses one or two key conversions between metric and American to compute the rest.
You can also run it directly as an application by downloading the jar file, saving it to a directory, and then opening a terminal window in that directory and saying:
java -jar metricConverter.jar
If clicking on the below links fail, please try holding down the shift key while clicking.
For more information on metric conversions, go here... No, just kidding. Try the National Institute of Standards & Technology, hopefully a good starting point:
They provide an exhaustive(ing?) list of conversions in a .pdf file, "special publication 811". (if this link yields a bunch of garbage text, try holding the shift key down while clicking on it)