This blog has some nice resources for a study of the explorers of the world. I particularly love the Explorers Cards!
Imagine of Leif Erickson for timeline
Image of Christopher Columbus for timeline
Image of Hernán Cortés for timeline
Image of Francisco Vaquez de Coronado for timeline
Image of Ponce de Leon for timeline
Image of Hernando de Soto for timeline
Conquistador coloring page
Teacher Oz has a nice list of links for Exploration and Discovery.
The Mariners' Museum also has some nice resources on Exploration Through the Ages.
We hear so much about European explorers, but what about explorers from other parts of the world? Chinese mariner Zheng He (Cheng Ho)
American Journeys is full of eyewitness accounts of exploration in North America for the older student.
Social Studies for Kids has lots of good info about a dozen or more explorers.