I had a bunch of wine bottles in my window, and plants in the other window. But then my plants started having seedlings, and I didn't have a flower pot for them- and my wine bottles looked lonely without anything in them.
Suddenly, it hit me! Wine bottles could be flower pots, thus solving all of my problems! And it looks really cool in my window to have some bottles full of water, some empty, and some full of plants.

I did the first one less than a week ago, so no promises on the results (long-term, I'm pretty sure this isn't good for a plant), but right now it is so cool that I had to share.

UPDATE 3/10: Growing great! And I planted a few more. See last picture in this step.

Step 1: Get a Bottle.

Get your wine bottle. Clean it out, because alcohol will poison your plant.

Step 2: Put Some Gravel in It.

Fill your bottle up a few inches on the bottom with fine gravel- it has to be small enough to fit into the neck, but other than that the bigger the better. This is for drainage.

Step 3: Put Some Water in It.

Just because it will be hard to put water in it afterwards. Fill it up to around the top of the gravel.

Step 4: Put Some Dirt in It.

Fill up your bottle! Don't pack it in too tightly (the roots will have to go through this stuff), but poke it down with a stick or something so that it doesn't have air pockets that will make your dirt collapse in on itself later.
Fill about halfway up the neck.

Step 5: Plant Your Seedling!

I used a sprouting seedling that I already had.
This would also work with a seed.
Just pick a plant that seems willing to put up with living in a wine bottle.
Best if its root structure is straight down and not broad.
Put some more dirt around it so that it will stay.

Pictured are the seedlings and parent plant that I used. I have no idea what plant it is, but it doesn't die easily.