Set is a puzzle addiction of The Endless Thread.
There are practice games at the New York Times and thebreretons.com. TET's canonical daily puzzle is at setgame.com.
If you have Greasemonkey or Opera, there is a user script which removes the popups that say "GREAT! 1 Set Found" and "You already found that one! Still have 5 to find".
(If you try the script on Opera or Chrome, please leave a success report or bug report here, either directly on this page or at Talk:Set. It is known to work with Firefox, expected to work with Opera, and may work with Chrome.)