Wikipedia, which turned 7 this year, is a source of information for 683 million visitors every year. A poster child for user-generated content, Wikipedia has grown from its first year in which just 12 articles were created to over 10 million today in 253 different languages. That's a whole lot of content, and naturally, being able to easily search it would be helpful for anyone wanting to get the most out of the web's favorite encyclopedia. You could use the site's official search engine, or you could search Google for "site:wikipedia.org" ... or you could use one of the 10 alternative methods below (in no particular order).
Powerset is a much-hyped semantic search engine that uses natural language processing to "understand" concepts in web content and match pages to queries. Right now it only searches Wikipedia (and Freebase). We put it through some early paces last week.
Wikiwix calls itself the "ultimate Wikipedia articles search engine." It searches all of the Wikipedia sites at once (i.e., Wikiquote, Wikiionary, Wikinews, etc.) and has a very handy Wikipedia image search.
AskMeNow is a mobile-targeted Wikipedia search engine that does some natural language processing similar to Powerset and then attempts to cull your answer directly from Wikipedia. Like any NLP search, it's not perfect, but often enough it is right on the nose.
Similpedia lets you find related content on Wikipedia. Paste a URL or a paragraph of text and it will dig up articles on Wikipedia that are in some way related.
Gollum is a Wikipedia browser that supposedly "[reduces] the complexity of information" and makes it easier to browse the online enclyclopedia. To be honest, though, we can't really see any benefit over just browsing Wikipedia in Firefox.
Qwika doesn't just search Wikipedia -- it searches wikis. 1,158 of them. Wikipedia is included in those it searches, however, and the site makes it easier to search across multiple languages.
WikiMindMap is one of the coolest Wikipedia search mashups out there. Enter a search term, and the site will generate a mindmap based on related Wikipedia entries allowing you to easily explore a topic and its related articles in full.
Wikiwax gives Wikipedia search the AJAX suggestion treatment. Get search suggestions while you type and find that Wikipedia article a fraction of a second faster.
Lexisum takes Wikipedia articles and summarizes them to a smaller, more digestible format that are better set up for printing. You can choose from a number of standard print sizes to display your article summary (A4, A6, etc.).
Ask.com and SearchMash (a test sandbox for Google) each augment their search results with information from Wikipedia. Not a pure Wikipedia search, but interesting stuff from a couple of major search players.
Bonus site: Wikirage
Wikirage is something like Google Trends for Wikipedia. The site shows trends on Wikipedia based on edits. Hot this week for example, the Sichuan earthquake and American Idol. We gave the site a full review last August.