Edudemic.com just updated their list of the the 100 most powerful search engines. They include a number of categories (general, meta, databases and archives, journals and books, science, math and technology, and you guessed, history.) I've listed a few of the links below but if you go to the site you can find all of them. Some are really good.
General Search: Iseek. Edudemic described it like this: "targeted search engine, designed especially for students, teachers, administrators, and caregivers. Find authoritative, intelligent, and time-saving resources in a safe, editor-reviewed environment with iSEEK." I treid it and really like it.
History: History Buff: online newspaper archive with free primary source collection.
Genesis: "A dataset with descriptions of women's history collections from museums, libraries and archives in the United Kingdom and a Guide to sources that provides access to a wide range of international web resources on women's history."
History and Politics Outloud: "a searchable archive of important recordings through history, particularly politically significant audio materials."