This is an "alpha" release: it's had minimal testing. It's in Java, so it shouldn't be able to break your computer, but it's certainly not guaranteed to work.
Polymetronome is available as a Java archive file. It's less than 90k in size, so it should take only a few seconds to download.
Step 1: you will need the Java runtime environment version 1.5 or newer installed on your computer for this to work. It's possible that Java is already installed on your computer. To check, visit the web page http://www.java.com/en/download/installed.jsp and click on the button that says "Verify Java version". If you don't already have it, then you can install Java by following the instructions on that page. It's free of charge. (Note that you don't have to download the newest version if you don't want to. If you already have version 5 or newer then Magic Metronome should work on your computer.)
Step 2: download polymetronome.jar, save it to a folder and remember which folder you put it in!
Step 3: If you're using a Macintosh or Windows computer, you should be able to start Polymetronome by going to the folder you used in step 2 and double-clicking on the new file.
(Sometimes this fails to work on Windows Vista. It may be possible to fix it using the Jarfix application. Or you can start Polymetronome from a command line as described below.)
(Step 3a: If you're using linux, then you will probably need to start Polymetronome from the command line. Go to the folder you used in step 2 and type the command "java -jar polymetronome030.jar".)
If you have any trouble installing or running Magic Metronome, then please send me a message with details.
If you want to laugh at my clumsy source code, or if you want to make constructive comments, it's all in the file polymetronome-0.3.0.tar.gz.
Please send me an email with your comments and suggestions!