Yahoo Y!Q contextual searching

As best I understand, you give it some context (a sentence, a piece of a web page) and it will try to figure out the context and thus related stuff, anyway, there's lots of bits to play with.

Introducing Y!Q beta

including: use it in Yahoo News, an IE toolbar, various ways to add it to Firefox, and even embedding Y!Q on your own site

via Slashdot Yahoo's Y!Q Contextual Search Beta /.