The people at RLG (Research Libraries Group) have put out a free (no ads either) search service at
http://www.redlightgreen.com/

The search runs over about 120 million library book and article descriptions. Matches are weighted by, among other things, popularity in libraries so you're more likely to be able to find matches higher up on the results list.

For any single result you can get the info out in popular citation formats including MLA, APA, etc. This alone is a big boon for students and other paper-writers.

You can search by keyword, author/title, or ISBN. Advanced search permits non-English and year of publication specification.

At the very least it's a sophisticated Web card-catalog application that I would have loved to have as an undergrad. I haven't played with it enough to know what else is under the hood.

Personal, non-commercial use only; no searchbots.
This application brought to you by the public domain and publicly funded libraries.