I'm working on a book about seahorses, Poseidon's Steed by Helen Scales (Gotham, October 2009), and found an interesting Web site in the book's endnotes.
The book says that every two years the World Conservation Union (IUCN) enlists expert biologists to assess the status of species (animal, plant, invertebrates, birds, etc.) that are under threat of extinction. Each species is assigned a threat category, from "Least Concern" to "Extinct."
I challenge you to find a full list according to threat (I couldn't find one), but the home page has a running slide show of select species and their threat categories. There's also a page dedicated to some species that are considered threatened. I did not know, for instance, that the black-footed ferret was once considered extinct in the wild, that it has been reintroduced into the wild, and--though it is still considered Endangered--its numbers are considered "self-sustaining" in South Dakota and Wyoming.