I've known this for years as the sauce on my gyros. It's great for so much more, though!
It's a Greek cucumber garlic yogurt dip. Pretty low calorie. You can buy it at the store or make your own. You want to use the Greek yogurt, which is thicker. I've strained my regular yogurt before to be thick enough.
Sometimes it's nice to just spread a pita with tzatziki as my snack. I put it on my baked potato - nothing else, just this. It works well as a veggie dip. I need to make up another batch soon - mmmm. You could use it as a sandwich spread.
What else do you like to use tzatziki on?