One of the great revolutions of the internet age is that an increasing number of books are available for searching online, books on all topics, and from all eras since publishing began. Some of these books may well have information about your family and your ancestors in their online pages.
Start your search with Google Books (of course!). The familiar Google interface makes searching easy, though the search results will be less familiar. Because of varying copyright issues, some books are presented only as snippets, while others appear in full text (you can even download them).
Anyone you know?
For instance, you can download the 500 page Rocky Mountain directory and Colorado Gazetteer, for 1871, or simply search it online. With some clever searches, you can find many other city and regional directories.
Amazon.com also allows searching in the content of books. Their collection leans towards more recent publications, but is still worth a look. Unless you're searching for a highly unusual name, you should put the name in quotes, to avoid a ton of spurious results.
Questia bills itself as the World's Largest Online Library of Books. Whether true or not, there's a lot here. It's a subscription resource, but you can search for free, and get valuabe snippets of the results. They also offer a free trial worth checking out.
Next, visit the World eBook Fair, a large collection amassed from Project Gutenberg, the Internet Archive, and other deep resources. Best thing about them is an excellent search interface (all too rare, I'm afraid). They do charge a small fee to join the site, but searching is free, and you can usually find any books of interest at other sites, at no charge.
Lastly, and sadly, I feel compelled to mention the Universal Digital Library, more popularly known as the Million Book Collection. And yes, there really are a million books online here. This should be one of the internet's (one of the world's!) great treasures, but it's so poorly implemented as to be almost useless. Still...might be worth a browse if you've nothing better to do.
Visit the main page of Free Genealogical Tools for more, umm, free genealogical tools.
And don't forget to also check for your family history at Ancestry.com and NewspaperArchive.com.These are subscription databases, but they are among the most powerful research tools available for looking into family roots.