The Perseus Digital Library
The Perseus Digital Library contains 19th-century books on California and the Upper Midwest and Chesapeake regions, and a 19th-Century American Collection containing several genealogically valuable texts. A simple search function is available for all of these collections, but for the last you'll find the just-released "Named Entity Search" much more useful. An automated system was devised to extract person and place names from the texts, and even dates. Type in a date and see what was happening the day your ancestor was born, married, or died. These search functions are still being tested, so be prepared for occasional frustration.