There are a couple types of cozies on the market for sale. Some insulate bowls, cooking pots or are made for freezer bag style.
We carry a UL Fabric Cozy on our website. They are handmade Sarah in Washington State. They are flexible, washable and breathable, designed for quart freezer bags and Food Vac bags. They can also be used for items such as Enertia Trail Food and Mary Jane's Organics. They were featured in Backpacker Magazine's "Be An Ultralight Gourmet" December 2007, as part of their UL kitchen setup.
AntiGravityGear carries quite a number of styles of cozies, both for freezer bags, Enertia food bags, bowls and for insulating your pots if you cook in your pan. The cozies they sell are soft sided and UL. The only downside is they are not compact for carrying.
Wilderness Dining carries a light cozy for freezer bags similar to AGG.
You can use a number of items to make a cozy. You can use a fleece jacket, a hat, or whatever extra clothing you have laying around (though, if you are a true light packer, you don't have a lot of extra clothing laying around!, and in bear country, do NOT use your clothing.)
Eventually you might think, I want something only for my food. You can do anything from sewing two potholders together, sewing a fleece pouch, taking a pipe insulator (the ones you use outside on your hose faucet). These all work great. The main point of a cozy is to keep the heat in your freezer bag, so that your food doesn't cool too fast, and it rehydrates properly.
The outside faucet cover is one the best cozies you can make for winter & cold temperature use. It is bulky, but it is worth the space taking, you can fit your gear into it in your backpack, and it holds a quart freezer bag perfectly, and holds it upright, so you have a "bowl" for eating. This makes for less mess, and easier eating.
This cozy was made for me by Marszit. It is about the best I have found - it sits flat, has a shell to protect the foam inside. Its downside is that it isn't very compact.

For summer time, I like using Fozzil or Oriksa Dishes. They are folding dishes. They are light, affordable and pretty nifty. The plate and bowls fold up easily and makes a great summer time cozy. The bowl holds a quart freezer bag nicely. If you have small children a stable cozy or bowl will help your child eat with less mess or risk of dropping food on themselves.
Sgt. Rock's DIY Country Time Cozy is an easy to make multipurpose cozy. It can be used as a cozy, eating bowl and mixing bowl. You can use a number of brands from Country Time Lemonade, Kool Aid and Tang to make it. To use as a cozy, put your bag inside the "bowl" and add the water. Seal and do as usual. To eat, roll the bag top over the side. The "bowl" gives you a firm hold. In warmer areas you don't need to do the outside insulation either.
Make Your Own Cozy by StoveStomper is a pictorial on making a DIY cozy. You use Relectix material to make them.
Comments
Cozies
Has anybody tried to use Squishy Bowls for a cozie? I'm wondering if they would work. There rated up to 400D F.
cozy
I wonder if the insulated cover for a Nalgene bottle would work. If it keeps things cold, perhaps it would keep things hot?
Yes
The cozies for nalgenes work for both hot or cold. Often they are neoprene or similar.
I demand recognition
...of the padded tyvek mailer bag as the epitome of cheap, light, and effective cozies. :D As you yourself have pointed out they are additionally multipurpose, serving as sit pad, tent doormat, and padded stuff sack. :)
Why yes....
The padded mailer is the ultimate frugal penny pincher multi-purpose tool for hikers. And can also be used as a signaling device if in yellow ;-)