Symbolab.com (click)You may actually write an equation and/or text and it will try to find pages with equations that are logically relevant for your query.
I haven't been able to write a query that would produce a nontrivial result i.e. a hard-to-find page or paper that I couldn't easily locate with the text-based Google search engine. But if you manage to construct such an example, I will be very happy to learn about it.
At any rate, I am immensely impressed at least by the What-You-See-Is-What-You-Get (WYSIWYG) equation editor that is a part of the website. It would be nice if Blogger.com allowed me to incorporate a similar editor, so that the maths would constantly get translated to MathJax.
See a fresh interview with the father of this engine at TheNextWeb.com.