Wikia: Open Search, Open Source
You may not have heard of Wikia Search before. A relatively new project from the Wikipedia folks, they've entered the search-engine fray with an attitude that is just about as open as it is possible to be. Not only is their underlying software open source (under BSD licenses), but the very search results themselves are as open to editing as Wikipedia pages.
On the software side, Wikia depends mainly on Grub and Nutch. The former is a distributed web crawler, the latter a modular search engine that grew out of the Apache Lucene project. You can find links to these, as well as to Wikia's own user interface software, at their "Get Involved" page.
What makes Wikia truly interesting, though, is their attitude towards ensuring accuracy and relevancy of search results: let users fix them. After searching for a term, you're presented with the results and editing tools. You can add your own sites to the results, delete existing results, mark results as featured, edit descriptions, and use a rating system to move results up (or down) the list. Just as with Wikipedia pages, this activity is tracked on a sidebar, and edits can be undone by other members of the community if you're being a jerk.
Right now, Wikia Search is in a state that might best be characterized as "early beta": it works, but the results it returns are generally less comprehensive than those returned by Google or Yahoo! It will be interesting to keep an eye on this project as it moves forward, as an example of community governance - or as an example of a community overrun by spammers. I certainly hope the former triumphs over the latter.