Pomegranate molasses features in a lot of Middle Eastern dishes, and also in one very tasty cocktail created by yours truly. You can find it at ethnic groceries, or the trusty downtown Spec's, here in Houston, or you can make your own.
4 cups pomegranate juice
1 tablespoon lemon juice
If you want the molasses to be a bit sweeter you can add sugar- between 1/4 and 1/2 cup. I like it nice and tart, so the no-sugar version is what I use in my drinks.
Combine the ingredients in a saucepan. Simmer over medium-low heat for about 70 minutes, until the molasses begins to thicken just slightly. Don't worry that the molasses isn't super syrupy at this point - allow it to cool for about half an hour and it will achieve the proper consistency. (I say "slightly" because if you wait to take it off the heat until it's reached molasses consistency, it will harden into a delicious pomegranate brick.) For me, by the time the mixture began to thicken it had been reduced to a little less than 1 cup. (A saucepan with measurements marked out on the side is super useful for this.) If you're adding sugar, reduce to about 1.5 cups.
Once the molasses is completely cooled, store it in the refrigerator. It will keep for up to six months.