There are no standard Javascript APIs to get the mac address.
You can get this information using the Java applet, but you will need to sign it and have a policy file in order to access the hardware information, and it will not be really transparent to the user. See Obtaining a MAC Address on a Web Page Using the Java Applet
If you just need to keep track of the client, the most reliable thing you can do is use the API combination available by Javascript to create a persistent cookie. You can create a very persistent cookie using a combination of standard HTTP cookies, session storage APIs, caching tricks, and plugins. Evercookie is a great example: http://samy.pl/evercookie/
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