Questia, which focuses on the humanities and social sciences, claims to be the largest on-line library with more than 400,000 titles (books, journals, newspapers, and magazines). While fee-based on-line libraries have been criticized for aggressive marketing to students, Questia does offer some advantages over traditional libraries: they are open 24 hours a day, the contents of their holdings can be searched, there are no late fees, and they offer the means to electronically make notes in the works. Nonetheless, they say, "our service is not designed to be a substitute for a traditional library but rather is designed to make an extensive collection of titles and research tools available online to students 24 hours a day, 7 days a week." Their economics holdings seem to be fairly extensive.

Two of their most interesting features are automatic bibliography generation and full-featured searching, which includes searching the contents of works (this would seem to make it a powerful plagiarism detection tool -- indeed, this can be done free of charge). Note that non-subscribers have the ability to do complete searches. One significant restriction of Questia is that you can download only one page at a time.

Currently, they charge $19.95 a month, with reduced rates for longer periods.