NOAA
Noaa: Threatened, Endangered & Depleted Seals & Sea Lions
Resource provides information about endangered and depleted seals and sea lions. Great photos of the animals and clear, readable information.
Bagheera
Bagheera.com: Extinct in the Wild Caribbean Monk Seal
Though short, this article brings up some important points about extinct animals and how related species may go the same way. Be sure to read the "Questions for Thought" at the end.