Exactly which databases you'd use would depend on your topic. The Databases, eResources, & Tools list can be sorted in several ways, including by subject. For example, here's the History database list. The list below is a sample of those most likely to be of use to those in history looking for secondary works. There is also a list for Primary Sources.
Contains over 48 million records, covering all materials cataloged by OCLC member libraries around the world.