European Explorers: http://www.cdli.ca/CITE/explorer.htm
Explorers from Enchanted Learning: http://www.enchantedlearning.com/explorers/indexm.shtml
Hall of Explorers: http://virtualology.com/hallofexplorers/
Explorers at Yahooligans: http://yahooligans.yahoo.com/school_bell/social_studies/history/by_subject/exploration/explorers/
Of course, you can use Grolier Online too.
(Click on the desktop icon at school or use the Grolier link from home.)