This is a great multi-subject database that includes quite a lot of history content. It's very easy to use, especially if you try the Subject Headings. It's a good starting point for almost any subject.
Very similar to Academic Search Complete, but with an American history focus. Some of the material here is indexed but not archived (that means it tells you about the article but doesn't give you the actual text of it), so make sure you're aware of how to find the full text or order it for free via interlibrary loan. (Don't let those scary library terms intimidate you: it's quick and easy!)
This is the Big One. Full-text only, a massive collection, with a strong humanities focus. The downside? It's not as easy to search. Brush up on your keyword searching and search limiters (ways to narrow your search, such as by date), and your booleans (AND, OR, and NOT), and be prepared to try many different searches and approaches. Stick with it and you can find gold.