The ultimate guide for students who don't want to carry around their text books but still enjoy marking them up. With over 75,000 textbooks, Questia is an explosive host of information for the weary researcher. I really enjoy its easy navigation, ability to add comments in books, and wide range of topics. For an annual price of $100, I cut down on my book expenses and killed some book clutter in my apartment.
This is a free website that allows users to search for books not only by the title or author, but by ISBN numbers.
I found this helpful for when textbooks are hard to find. Instructors usually provide the ISBN numbers in their syllabi and this is one resource a student could use to locate out-of-stock books.