I never realized there were packaged products. I use dried porcini mushrooms frequently in various dishes, and I always strain and freeze the lovely byproduct of reconstituting the mushrooms in water: a truly wonderful porcini broth.
But do you have a specific recipe in mind? I notice that the items mentioned in the replies so far have additives (crucially, saltiness) that make the broth something more like a soup base (just add veggies or whatever, and you have soup). It's even something you might drink straight.
But my simple and unsalted mushroom liquid is really something else, more like a stock (it can be reduced profitably). I used some last night, in fact, as part of a lamb shank braise. Like fish sauce, it's a subtle, wonderful additive, in measured amounts.
You can make it quite easily from adding mushroom stems and trimmings to a basic vegetable stock base, along with a bit of soy, as CarrieWas218's GH link suggests. I use a good flavored (well, aren't all mushrooms with good flavor) wild fresh mushrooms, crimini, portobello, oysters, or whatever the market has, and definitely dried porcini, porcini powder or shiitake stems, but even culitvated (white or Parisian) mushrooms will impart a smidgen of mushroom flavor. Mushroom soy sauce is an even better add in or option for a soup base. Pearl River Bridge makes my favorite mushroom soy sauce.
Stock made with mushrooms.
You can buy it: http://www.google.com/search?q=mushroom+borth&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a#q=mushroom+broth&hl=en&client=firefox-a&hs=aTD&rls=org.mozilla:en-US:official&prmd=ivnse&source=univ&tbm=shop&tbo=u&ei=P-SdTYOYLvGD0QGwlfDPBA&sa=X&oi=product_result_group&ct=image&resnum=3&ved=0CD0QzAMwAg&biw=1920&bih=950&bav=on.2,or.r_gc.r_pw.&fp=27105db9f40b6df9
Or Gordon Hamersley's version with soy sauce: