A hoatzin. Hoatzins are awesome not only because they are "flying bioreactors", but also because they are a bit like modern-day versions of Archeopteryx, the ancient gliding bird ancestor which had claws on its wings which enabled it to climb up trees. Hoatzins live in the Amazon basin. Image courtesy of www.birdsofseabrookisland.org.
Both animals have a wide array of bacteria which make cellulase and lignase enzymes the host animal cannot. These enzymes break down leaf components such as cellulose (long strings of glucose linked together) and lignin (the irregular, phenolic (or ringed) compounds which give the leaf structure), which the microbes ferment into short-chain fatty acids such as butyric acid (which gives Parmesan its "distinctive" smell), propionic acid (which smells like really bad sweat), and acetic acid (as in vinegar). Because this process is relatively slow, the animals must eat a lot of food and have a large fermentation chamber; hoatzins are poor flyers and have to have an extra bump on their chest to help balance on branches so their full gut doesn't topple them, and the cow rumen is so big you could probably fit an adult human in it, though I don't think anyone has tried it.
Compare how much space the crop - the pouch birds use to store food if it over-gorges itself - takes up in the hoatzin (left) compared to the chicken (right). This is where the "pre-digestion" of vegetation occurs in the hoatzin. Small amounts of fermented fluid are released into the small intestine where the short chain fatty acids can be absorbed. Pictures from schaechter.asmblog.org and keep-hens-raise-chickens.com
Cows and hoatzins aren't the only animals which depend on microbes to break down their food. We too depend on microbes, except the majority of our microbes live in our large intestine and feed on our "leftovers" because most of our nutrients are absorbed in the small intestine. Research indicates that some other organisms, such as the giant panda, have lost some of the ability to degrade complex plant matter, and their genomes contain fewer genes encoding enzymes involved in this process than their nearest omnivorous relatives. This might explain why there have been reports of mother panda's feeding offspring their feces - populating your gut with the right microbes is obviously important if you cannot digest your food yourself.
Of course, pandas aren't the only animals to practice coprophagy (poo-eating). Babies do it. Dogs do it. And rodents like rabbits and guinea pigs do it. The last two animals are relatively easy to explain...they are hindgut fermenters, which means the majority of the microbes responsible for breaking down the plants they eat live in a part of the gut which comes after where the majority of absorption occurs. Therefore, in order to get all of the nutrients out of the food they have taken in, the food has to pass through the gut a second time. But babies and dogs...let's just say I don't kiss them.
If you want to learn more about poo, Wikipedia has your a** covered