MashProxy is an open source project, released under the GPL. You'll need CVS to build your own copy of the Java applet if you want to run it on your own website, since my applet is hardcoded to only work on my domains. The sourceforge project page gives directions on how to get the code.
If you just want to experiment on your own machine, then life's a lot easier. Download these files:
If you have them all in the same directory, and open index.html in your favorite browser, you should see the usual start page appear.
You can then start playing with searchgui.js, making alterations, or even write a whole new page structure and script using the mashproxy applet.
You won't be able to run the applet on your own website though, for security reasons I didn't want code I'd certified as safe being scripted by anyone else. To distribute it yourself, you'll need to get a certificate, build the java project yourself, and sign it. That's a complex process, and will have to wait for a future entry!