This list of URL's is a growing list of what I personally consider to be really useful to any person exploring geology, particularly vertebrate paleontology. This is in no way intended to be an exaustive repository of internet resources, only a springboard for imagination and a reference point for research in the field.
Dinosaurs:
Paul Sereno : Paleontologist - Dinosaur Expeditions and Discoveries in Paleontology
Dinosaurs: The Natural History Museum's Dino Directory
Mammals:
John Day Fossil Beds National Monument (National Park Service)