At times, I've found that Wikipedia's internal search engine is sometimes a little bit lackluster: on occasion, I legitimately can't find what I'm looking for. This will probably improve somewhat when Wikipedia founder Jimmy Wales launches his Wikiasari search engine. Though it's apparently going to be a search engine for the whole Web, not just Wikipedia, I'm betting that the technology could have an effect on improving the online encyclopedia's internal search capabilities.
Or there could be a third-party search engine that gets there first, which could be the case with WikiSeek. Created by start-up SearchMe, this just-launched search engine's results consists only of Wikipedia entries and the sites that they link to. It might not be an official part of the Wikipedia family, but it was created with the encyclopedia's permission and assistance, and the majority of site revenue will be donated to the Wikimedia Foundation. There are also some features that you don't necessarily see in conventional search engines: a tag cloud of Wikipedia categories at the top of each search page, for example. Additionally, you can install a Firefox extension to put a WikiSeek button right onto Wikipedia's home page.