Here are the sauces that I make regularly using plain yogurt:
1) the tzatziki that you mentioned
2) raita - which is very similar to tzatziki depending on how you make it... drained yogurt, drained grated cucumbers, cilantro, salt, and whatever spices (I usually use a little mint and a decent amt of cumin; my MIL would probably put in cumin & red chili powder) - served with any Indian/Pakistani food I make
3) yogurt tahini sauce from the falafel recipe that's on the blog:
1/2 cup plain low-fat yogurt
2 tablespoons fresh lemon juice
2 tablespoons tahini (sesame-seed paste)
1 garlic clove, minced
Yogurt usually lasts a while, so you could use the rest of it next week by making some Greek-style grilled chicken and putting it in pitas with tzatziki. You can substitute sour cream for yogurt in a lot of things - even on a baked potato. You can make a "curry" chicken salad - cubed chicken, celery, apples, dried cranberries, with a sauce of drained yogurt, a little mayo, & curry powder. Also you can bake lots of stuff with yogurt. There are a bunch of recipes on Cookinglight.com. I know I have made some CL coffee cake or blueberry coffee cake (or maybe both) a few times with yogurt and it was amazing, but I can't remember which one it was right now.
And another way I use yogurt is to thicken sauces in Indian dishes. The two that I'm thinking of right now are:
Ground beef (actually should be lamb, but I'm not a lamb cooker) with garlic, ginger, onion, diced tomatoes, spices, & yogurt. I've had ppl tell me to make it a number of different ways, but the simplest on was where you brown the beef & onions with all the spices, garlic, & ginger and then throw in the tomatoes and the yogurt toward the end. The basic spices are turmeric, cumin, & red chili powder. You can add other stuff like cloves, cinammon, cardamom. It might be just as easy to use a premade garam masala and some extra chili.