Browse around the Dewey number 910 and 970 for general books on explorers.
The Age of Exploration - Information on most of the important explorers, from the ancient world to Capt. Cook. Created by The Mariners' Museum in Newport News, Virginia, USA.
Discovery and Exploration - Library of Congress American Memory site, which includes images of explorers' maps, journal entries, and historical documents.
PASSAGES--A Treasure Trove of North American Exploration - From the National Library of Canada, this site had articles about many North American explorers, including many lesser known French explorers of Canada.
Maps of the Explorers - Maps of explorers, de Champlain, Hudson, etc. from Empire of the Bay, a PBS production.