I've used gallon zip lock bags and they work fine. You leave about 1 inch of the zipper open for aeration. You need to put a few holes in the bottom for drainage and some people use a clothespin on the top to keep the aeration hole open. They stand up fine because of the weight of the moistened soil. There is a little less surface area to sow on compared to a milk jug but otherwise they are good!

I had problems with the bag tops folding over and used bamboo skewers to keep them up. Poked the skewers through the upper part and into the soil.
Also preferred using quart sized bags. The seedlings were planted out before getting very tall so height was not a major concern. If you want to grow on seedlings in the bag when the weather is warmer just fold down the tops.

Placing the bags in a container helped when it came time to move them.

The last few years I have used milk jugs, 2L bottles, 3-4" pots in clear plastic containers/flats.

All the above can be reused, including the baggies.

I used small sammitch bags for my first try sowing, and they are sprouting like the dickens. Only problem is, the tops of the bags are folding over and I'm afraid it's going to kill/stunt the seeds. Will it? Do I need to take some action, or just chill out (Ha! Winter Sowers Chill Out!). If I do need to do something, what is it? I see the advice about bamboo skewers, but I have 70 little bags to skewer! Does anyone have any other suggestions? Help!

If you have other containers, you could place all the baggies in it, cut off the bag tops, and use a top for the large container.

dikobraz, I used the bamboo skewers that I got in the grocery store 100/$1.
They worked but were putzy. Decided that if I were to use baggies again I would use a cardboard collar that is made to fit inside the baggie to hold the top open. Think it was a posting by Monte last spring. Just cut a piece of cardboard to fit the baggie and tape it into a circular collar then put it in the baggie. I think it would work just to sit on top of the soil so that you do not have to disturb the seedlings roots or could be pushed into the soil.

If you try that, PLMK how it works.