A newly launched service called Wikiseek focuses on complimenting Wikipedia by restricting search results to articles and references in the encyclopedia. Wikiseek's about page claims that this method makes it "an authoritative source of information less subject to spam and SEO schemes."
Wikiseek suggests search refinements "based on user tagging and categorization within Wikipedia." When I do a search for the word "apple," Wikiseek returns a category cloud displaying categories which which my search phrase appears most often. The "Apple hardware" category shows most prominently. Actually, I was more interested in the fruit because I had one with lunch today. Clicking on the fruit category gives me a list of those references in Wikipedia that refer to "apple" as a fruit. Pretty nifty! If I go back and choose "Apple hardware" I see the expected list of articles about current and legacy Apple products.
There are some instances of strange results, such as the search for "wiki," and comments at the TechCrunch article are quick to point out that since Wikipedia is editable, article spammers now have more incentive if their actions will effect a Wikipedia-specific search engine.
Personally, I find the category refinement feature and reference results useful although not enough to build an entire site around. It would be better utilized as a module in a large package... perhaps Wikipedia itself.