I believe the idea of most supplements is to keep it in your system. Your body is using the supplement to rebuild/repair muscle even on your off days. Remember, you tear down muscle when you are working out and you make your gains when you are not working out. With creatine, you have a loading period, maintenance period and an off period. The label should explain this.
Personally, I take a shot of creatine before working out followed by a post workout whey protein shake. After my evening meal I sometimes mix in some creatine with my nighttime protein shake. Creatine is relatively cheap so I don't worry about the cost too much and I do take it even on off days.
You will get different recommendations here. The best advice I can give you is to experiment and see what works best for you, but I take all of my supplements even on my off days.
The only supplement I can think of NOT taking on a non-workout day would be something in the line of a Nitric Oxide supplement like NO Explode or Superpump 250, even though their labels recommend that you do take it even on off days.
Hope this helps!