Buttermilk options.

Merrill's Saag Paneer (and you can make your own paneer while you're at it, using buttermilk, with Jennifer Perillo's Creamy Homemade Ricotta . . . just strain it longer, while pressing out the liquid, to make cheese. Don't forget to save the whey, for use in polenta, breads, creamy soups, etc.). I agree with everyone else's suggestions. Wonderful in cream of spinach soup; so, so easy, especially if you use frozen spinach. Also, keep in mind that buttermilk usually holds well beyond its "sell-by" date if kept in the back of the fridge. ;o)

Make cornbread and then pour some buttermilk and a drizzle of maple syrup over it while it's still warm. Heaven!

Great for making homemade dips. Add to cream cheese, put in some dill or dehydrated onion, parsely, etc. If you have no cream cheese sour cream works too or just use the buttermilk alone. It will be more runny but the kids won't notice if you cut up some carrots, celery etc and put it on a plate.

Good question, cause I'm also running out of ideas for the buttermilk I have leftover. I love using it in my go to recipe for banana bread: http://norecipes.com/blog/2009/02/20/buttermilk-banana-bread/

Use it in place of yogurt in Amanda's recipe for South Indian Yogurt rice

If you search 'buttermilk' in recipes, you'll find 30+ pages -- I just breezed through the first 5 pages and I'm excited. One that particularly appeals to me is the Buttermilk Pie with chilis! Here's one that I posted, which seems to me to get better every time I make it: http://www.food52.com/recipes/14441_new_england_pear_harvest_tart. Also scones, breads from AntoniaJames, and so on...
Once I started keeping buttermilk in the fridge, I've found that I love it. Tangy taste, easy to pour and measure and it keeps for a long time. You can use it in place of yogurt, making adjustment if needed.

Last time I had leftover buttermilk I made an "oven fried" chicken recipe that turned out great. Marinated the chicken pieces overnight in buttermilk, garlic, a little chopped up chipotle, honey and thyme (+ salt & pepp). Then breaded and baked. Deelish.

I tested a smoothie recipe awhile back that calls for buttermilk. It's delicious. http://www.food52.com/recipes/13086_good%20morning%20raspberry%20smoothie

Dressing for my "Dilled Crunchy Sweet-corn Salad" is good on any kind of salad; would work on a nice crispy Romaine salad for example:
http://www.food52.com/recipes/12928_dilled_crunchy_sweetcorn_salad_with_buttermilk_dressing

I make a Buttermilk Alfredo sauce that goes great in pasta or mashed taters! Add a couple of tablespoons of cornstarch to a quart of buttermilk. Leave that to sit while you sautee onion, mushrooms, and spinach. Pour in the buttermilk and season with salt, pepper, and any herbs you want. Stir until it bubbles (keep it on medium heat), then add a melty cheese. Serve on pasta or mix into mashed potatoes.

Also, smitten kitchen has an absolutely fantastic buttermilk cake. Google it.

It can be frozen for use in baking.

Muffins!!! Homemade ricotta!!

Should be MASHED potato (darn iPad spell checker!)

Based potato, soda bread, cornbread, pancakes, salad dressing (with blue cheese), scones, fried chicken (or pork cutlets) - dip in buttermilk then in seasoned flour and fry in hot 350F oil....etc.