You can copy, modify, and redistribute the source code, under the GNU GPL of course. It also has credits for the people who made suggestions and bugfixes.
To play, click on your player button, then click 3 cards that you think might be a set. Don't worry if you mess up - just try another 3. The cheat button marks three cards that make up a set - I wouldn't recommend using it in games with your friends :). The rules are available from http://www.setgame.com. Or try the "help" button, which has cool stuff. If you're alone, try the new single player mode.
I play with these settings and win about half the time.
If this version doesn't work for you, try an older version