Serious Eats - seriouseats.com

I definitely second (third, fourth?) the votes for mashed potatoes, pancakes/waffles, red velvet cake, etc., but I made these doughnuts from SE the other day and they were AWESOME:
http://www.seriouseats.com/recipes/2009/10/glazed-buttermilk-cake-doughnuts-recipe.html

Also, this chicken recipe (marinated in buttermilk) is outstanding:
http://www.epicurious.com/recipes/food/views/Spicy-Oven-Fried-Chicken-103512

And a great homemade cilantro ranch dressing recipe:
http://www.feedyourkids.com/2008/07/taco_ring_rice_beans_and_salad.html
(scroll down for the dressing recipe)

Here's a GREAT recipe for french toast from Gourmet (boo, hoo!) magazine (seriously, SO easy and really startlingly good):
http://www.gourmet.com/recipes/2000s/2008/11/buttermilk-french-toast

and a fabulous biscuit recipe:
http://www.feedyourkids.com/2007/01/easy-buttermilk-biscuitschicke.html

The red velvet cake recipe I use (adapted from Paula Deen's cupcake recipe on Food Network) calls for buttermilk.

I usually have a little left over - no matter how small a bottle I buy, so I like to make garlic mashed potatoes with little red potatoes and chives. The buttermilk makes gives a hint of tangy.

My husband will also drink it straight; so do I, when I remember. Our commonest use for buttermilk is RANCH DRESSING. We always make our own, never buy it. Half buttermilk, half mayonnaise, a splash of cider vinegar, a little salt and plenty of minced garlic. Herbs to taste. Fresh basil is good, but I'm thinking a pinch of chipotle flakes next time.

Bran muffins - and by the way, you can use dried buttermilk instead in the dry mixture - adding water to replace the liquid. The flavor is great. And you don't have to worry about what to do with the rest, since it will sit in the pantry and wait for the next time you need it.

@Chiano--interesting to think of sour milk in coffee! Don't think I could manage that one (as I prefer my coffee sweet and creamy and have had many a time my milk has gone over and I toss out the coffee if I added it before noticing.)

@ocarol--Oh my, buttermilk gingerbread sounds VERY good. I will have to try making some now that it is gingerbread season! YUM!

@betteirene--I would rather watch someone drink buttermilk than clobbered cream! Now THAT is hard to watch!

All of the above. I love buttermilk for baking.

My dad would pour it in a glass, sprinkle it with pepper and drink it. Can't do it. Shivers down my backbone!

Yes, I've always got a carton in my fridge and use it for all of the basics; Ranch Dressing, Biscuits, Pancakes, Waffles, Chicken (and other meat) marinade and frankly I find it to be a refreshing beverage on its own.
All of that said, my real comment is that while in Graduate School I traveled extensively with one of my professors and when we'd stopped in a diner for a meal at the end of the day, he'd invariably order a slice of cherry pie served in a soup bowl with a glass of buttermilk served on the side. That glass would get poured into the bowl with the pie and he'd enjoy every bite. Truth is it's not such a bad combination! It takes all kinds...

Seconding leek+potato soup--heavenly!

Oh my goodness. What great ideas.

I'm going to do some baking, then pick up some more to do a gratin.
I did make a small batch of ranch dressing and am thinking about making some buttermilk mashed potatoes with chives for tonight's dinner.

This is so great. Thanks for all the suggestions! I'm thinking buttermik should be a refrigerator staple instead of an occasional purchase.

@ag, as people get older, the gradually lose their sense of taste, and sweet is the last to go, which is why sometimes older folks will complain that nothing tasts as good as it used to, but they'll still like sweets. Maybe something like that is going on with grandma, but she's objecting to the sweet flavor instead of looking for more desserts.

Or maybe it just gives her something to complain about.

Well, after tonight's dinner I found out that there is something else I make that my FIL absolutely loves - latkes! Woohoo! It was actually really funny - we had plenty of food, 2 different kinds of latkes, all kinds of greens, salads, brisket, but all he ate was just plain old potato latkes (no sour cream, no homemade apple sauce, no homemade gravlax). The funniest moment was when dessert time came, and he said, very nonchalantly, not looking at anyone in particular, "I think I could use a couple more latkes" (it's good that I made a quadruple batch). He was the happiest I've ever seen him at the dinner table (and probably the least stressful dinner with him present)! I told my OH that from now on, no matter what the occasion is, I'm always serving latkes when we have them over for dinner:-).

@HeartofGlass, 40 minute hard boiled eggs?? Wow, sorry, but I find that fascinating. I'm trying to imagine the color of the yolks and well just the overall texture of the eggs.

Dan Barber, cos he knows his oats