See the link below for source and what is more or less a toy applet to demonstrate sniffing the MAC Address(es) of a machine from the browser. The HTML illustrates the simplest (albeit hackish) approach for cross browser support. I've tested this with FF 2.x, IE 5-7 and Safari 3.x on Windows, unfortunately Leopard doesn't support Java 6 yet. This solution will only work with Java 6, I do no checking in the code for this fact, it's a demonstration after all.

One thing is for sure, IE treats Java as a third class citizen, I've done some timings: FireFox averages  10.5 seconds to start the Java plugin (which is barely acceptable), but IE is terrible with an average startup time of 31.8 seconds this is after clicking through the two levels of dialog about how this applet is insecure and may destroy your system, "oh the humanity" of course this can be turned off, but the default for most folks is to try and scare the pants of you.

You can find the package and source here:

http://www.softwaresamurai.com/Agwego/mac/macaddressapplet-1.0.tar.gz 

They are still supported but Sun's documentation has gone to pot. There's almost nothing out there describing implementing applets with Java SE 6. Sure there are a lot of examples with 1.4, but the world has changed modern browsers and W3C have deprecated the applet tag, and if you're trying to do something like interact with your applet via javascript there's almost nothing discussing this and of course the old Applet tag is broken in "Modern" browsers, and although there are 20 examples they all seem to be toys with respect to Web 2.0, namely they are still using the applet tag, thanks for coming out.

Here's what I've come up with for embedding applets and interacting with them via javascript, it's a bit hackish but it has the cleanest/simplest code I could come up with.

    <!--[if !IE]> Firefox and others will use outer object -->
    <embed type="application/x-java-applet"
           name="mac_address_applet"
           width="0"
           height="0"
           code="com.yourco.MacAddressApplet"
           archive="macaddress.jar"
           pluginspage="http://java.sun.com/javase/downloads/index.jsp"
           style="position:absolute; top:-1000px; left:-1000px;">
        <noembed>
        <!--<![endif]-->
            <!---->
            <object classid="clsid:CAFEEFAC-0016-0000-FFFF-ABCDEFFEDCBA"
type="application/x-java-applet"
name="mac_address_applet"
style="position:absolute; top:-1000px; left:-1000px;"
                    >
                <param name="code" value="com.yourco.MacAddressApplet">
                <param name="archive" value="macaddress.jar" >
                <param name="mayscript" value="true">
                <param name="scriptable" value="true">
                <param name="width" value="0">
                <param name="height" value="0">
              </object>
        <!--[if !IE]> Firefox and others will use outer object -->
        </noembed>
    </embed>
    <!--<![endif]-->

Once I've repackaged the applet which "sniffs" the first MAC Address of any interface on your machine, which I guess could come in handy from time to time.