We've just updated Game Pixies with an unofficial SET variation called MetaSET.
We were introduced to MetaSET by a fellow SET geek at our local games night - the mechanics of the game work in the same way as SET, but the rules for identifying groups of cards are different. If you find SET a bit too easy, or want a new challenge, then give it a try. We have added some instructions to introduce you to the basic concept. You should probably know how to play SET first before doing this.
I was just having a dig around in the Wikipedia history for Set - at some point there was a list of SET variations on the page (removed because it was considered "made up"). In this list, MetaSET is called Super-Set. The SET website itself also lists some interesting variations.
You can play MetaSET on Gamepixies by visiting the main SET page and you'll see a new section there. Note that you'll only see it if you're using the new version of SET.
I hope you enjoy the new addition. It's multiplayer only at the moment, but we may make a time trial version in the near future.