It has been a long time since I did any khoj on the Internet. So here is one good one:
Clusty is a meta search engine. As the name suggests, Clusty clusters the search results. The best part of Clusty is that it is integrated with Wikipedia. So, you can just search for words in this search engine, see what the related categories are. In fact, it is not even necessary to open the pages. The results (especially in Wikipedia) itself give you what you are searching for.
If that is not enough, you have customization features, preview etc. There is also a Firefox toolbar for Clusty.