The History Place is a neat site where you can read articles and about World War II, the Civil War, or the Irish Potato Famine--and see pictures and illustrations along with them.  BBC has a popular history section on its site, and WWW.History connects you to online resources through a topical list you can browse or search.  And who doesn't like the History Channel? 
One more thing: If you want more, check out Best of History Web Sites, an award-winning portal to the top history sites by subject.