Old textbooks / SellTextbooks / CC2.0
Internet Archive : Books and Texts Archive - If you're researching something that has become the subject of history, you are likely to find something here. I found a great deal of information around Kent and Sheppey as it relates to the Norwood line.
Ancestry.com - This site was really indispensable to my research. I think there are lots of arguments for and against sites like this and I can see myself writing a whole post on it at some point. The Cons include the fact that folks share family tree information that is not necessarily reliable or well researched. However, overall, I love the site and where there are lots of members, there are financial resources to obtain and archive genealogy information - power of the masses, if you will. Ancestry.com has an extensive collection of birth, death, social security and census data, among other things. If you join and use their records, I recommend downloading the actual image of the records you use as you go so that you have them to refer back to later.
Google Books - Google books has a ton of books in electronic format. Often, you can only access excerpts of them, depending upon their copyright status. BUT, sometimes, an excerpt is enough.
Familyesearch.org - I really like that they make so many resources available for free. I understand the power behind charging a fee... you then have resources to offer more (hopefully). But the LDS church has resources and has opted to allow free access (without membership to the LDS church) to so many resources and I've found myself grateful so many times for that access.