Wired News has an article about Scirus. It's nearly a promotional item for the science search engine owned by Elsevier, but the author does make a point about why this for-profit publisher would make a search engine this useful available to anyone without a fee.
"The Scirus search engine is free, but that's not to say its owner, Elsevier, a giant in science publishing, doesn't benefit from operating the science search tool. Elsevier publishes 20,000 journals, books, electronic products, services, databases and Web portals.
Scirus includes Elsevier publications, like BioMedNet and ScienceDirect, in its results. Those results are ranked like any other, but also feature an eye-catching logo. Some results supply only an abstract, and require a fee to read the entire paper."