You could probably make some mean malt vinegar out of it. Just get some raw red wine or cider vinegar, get the mother out of it, and let the magic happen.

I love cooking using ale (or cider or wine for that matter), makes yummy bread and I use about 450ml at a time. Also, would be good for steak and ale stew or anything like that?

To echo the sentiment expressed above, I'd use it for a stew or pot roast. I use a can of Guinness beer in every crockpot of pot roast I make. It's delicious.

It should be great for marinating chicken before roasting too!

The Cook's Illustrated's Almost No-Knead Bread recipe calls for light-flavored beer, which doesn't have to be fresh. I keep opened beer in a sealed jar in the fridge and use it up 3 oz. at a time.

Beer bread. Nigella Lawson's beef stew.

My husband is an avid home brewer and this happens more than you would think. But don't give up hope too fast it might be able to be saved. There are usually two reasons for it being flat, a lack of active yeast or not enough sugar for the yeast to eat. You can buy some high quality yeast at a home brew store that is good in high alcohol beer and a little bit to each bottle and cap it back up. In 2-3 weeks if it still hasn't carbonated then you can purchase Muntons CarbTabs (they are small white tablets made of dextrose) and add them to the bottles according to the instructions.
If all else fails you can make carbonade. A traditional belgian stew made with beer and meat. However flat beer wont be so good for bread as it is flat and wont have the yeast or carbonation needed to lighten the loaf.

Happy brewing.

Onion soup, spicy mustard, or bread :)

Just today one of the other blogs I read told how to make your own beer shampoo. Reduce the beer to 1/4, let cool, and add to 1 cup of shampoo. Never tried it but beer is supposed to make your hair shine! My DH would kill me if I used his beer for shampoo.

Use it to steam clams, yummmmm

try this candy: http://www.sprinklebakes.com/2010/06/ale-and-pretzel-soft-caramels.html

I made them and they were delicious! A bit hit at the office too, there's zero alcohol content left by the time your done, but co-workers here that you brought in candy made with beer, and in a split second they're gobbled right up.

Beer battered fish!

add to chili or use as the liquid in bread. Both are Yum!

A technique I've used with a batch of homebrew I didn't like was to make an eisenbock / ice beer of sorts. Just pour the been into a freezer safe container for a few hours until slushy. The save the liquid and throw out the ice. It'll make a thinker brandy like drink out of the beer, but it's a fun option.

Keg it and force carbonate it. If you're not kegging yet, now is a good time to start.

Beer cheddar soup, yum.

http://www.barbayq.com/2011/01/budweiser-and-chicken-bff/

Beer-steam a whole chicken on a barbeque...

I use beer in all kinds of things that I could add beef stock to. I wouldn't use it for bread as Hurryupthecakes said. Contact your local homebrewers and try to save it before you totally throw it out! Many batches can be saved!

Beer cheese soup. I've been making variations on the recipe below for five years. (Including: milk instead of cream, any cheese I have available, different vegetables, barley in place of the potato) it has never been less than delicious.
http://www.midwestliving.com/recipe/soups/three-cheese-beer-soup/

They had this recipe over at thekitchn the other day:

http://www.thekitchn.com/thekitchn/candy/copycat-recipe-chocolatedipped-beer-marshmallows-139899

Beer marshmallows!!!

I'd use it in chili and, as others have suggested, beer/cheddar soup or beer/cheddar fondue.

You can stew sauerkraut, beer and pork together. Just do this on a Friday when you're sure not to have any guests for the weekend. You may not be good company for a few days.

Beer bread! Also my boyfriend makes an awesome pizza dough that uses beer as the liquid base.

Another suggestion is that you can mix it with a commercial beer or an overly carbonated home brew. Try variations until you find one you like. Sounds a bit crazy but can actually taste great and it not unheard of, for instance Newcastle is a mix of old and new ale blended together.

First, are you sure it's not carbonated? Especially since it's the winter, a lot of people's houses are so cold that it takes a long time to carbonate the beer. I bottled a batch in December that took over a month to carbonate. The key is to get the beer to at least 65 degrees. It also doesn't hurt to flip the bottles upside-down for a second to reincorporate the yeast. Don't give up on your beer yet!

If it is indeed not carbonated, you can actually still save the beer if you want. The recommended procedure is to dump all of the bottles back into your fermenter, pitch some yeast, wait a few days, add more bottling sugar, then re-bottle. As long as the beer hasn't been contaminated or anything this should work just fine.

I'm a homebrewer as well and if you're certain it's not carbonated then I suppose it's good for cooking some foods.

Some of the items mentioned can't be cooked with flat beer, including some of the breads, beer battered fish and perhaps some others. These rely on the carbonation that is already provided by the beer to create the lift (in bread) or the light crispy texture (beer batter) that you would desire. I'm no expert by any means, but that's what I know on the subject. Credentials include being a grad student and lots of beer consumption. :)

If the beer is on the darker side and not especially hoppy, I'd make beef carbonnade, a Belgian stew of beef, onions and beer.

A flavor profile would have been helpful, you know? Because if the beer is something like, say, IPA, it might make it harder to cook with.

I was also going to suggest kegging it. Or, you can get one of those smaller home carbonation set-ups and carbonate each bottle as you drink it.

Or, still, I think you could open each bottle, pitch some fresh yeast and sugar, and try re-priming. No reason that won't work.

Beer jelly! I am dying to try it.
http://puttingupwiththeturnbulls.com/2010/12/14/spiced-beer-jelly/

similar to ilovebutter, my first suggestion would be pop for a sodastream and carbonate it yourself. Don't see why it wouldn't work.

Otherwise, I'd reduce the hell out of it and bottle what I was left with to add to stews, baked goods, etc.

Beer bread, beer cheese soup, and spicy beer mustard (I've made all of these and they're all pretty freaking awesome).

welsh rarebit!!!!!!!!!!!!!!

When we brew beer I can't stand to waste anything- I freeze the wort and use it as a base for chili. Don't see why it wouldn't work with flat beer too! Also, I second the commenters who suggest another carbonation attempt (especially if it's really tasty and worth it). Good luck!

Do a 1:3 ratio with salted water and use it to boil lobsters.

re: the sodastream suggestions- I once made the mistake of trying to add more carbonation to a bottle in which I had already added the syrup- what a mess-