The Morae River was my first introduction to the concept of artificial ecosystems, but now I keep finding more and more about it. Apparently the internet is swimming with sites dedicated to things like alternate evolution, extrapolations of dinosaur development had they not gone extinct, and just full fledged alien worlds. Of course, a great deal of these are terribly illustrated, which, it seems to me, takes the whole fun out of it. There are a couple of bright spots though. The biggest is Snaiad, an alien world with a far greater catalog of fauna than the Morae River, with stunning and often disturbing illustrations, and a laboriously worked out anatomy. More so than the Morae River, it attempts to give a consistent and rational explanation for how unique evolutionary forces guided the creatures to where they are.
Other interesting works include After Man: A Zoology of the Future, and Expedition, which features the artwork of Wayne Barlowe. Of course, Avatar also features an extensive and detailed collection of original fauna, which you can watch on the big screen.