Best way to have script start up for all users in Mountain Lion?
Save a property list like this as /Library/LaunchAgents/test.plist:

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN"
"http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
    &lt;dict&gt;
        &lt;key&gt;Label&lt;/key&gt;
        &lt;string&gt;test&lt;/string&gt;
        &lt;key&gt;ProgramArguments&lt;/key&gt;
        &lt;array&gt;
            &lt;string&gt;say&lt;/string&gt;
            &lt;string&gt;aa&lt;/string&gt;
        &lt;/array&gt;
        &lt;key&gt;RunAtLoad&lt;/key&gt;
        &lt;true/&gt;
    &lt;/dict&gt;
&lt;/plist&gt;


It should be loaded automatically on the next login.

See man launchd, man launchctl, and man launchd.plist for more information.