Google SearchWiki lets you customize the search results by promoting, demoting and adding new pages. You can also annotate the results. Some people think that the new feature clutters Google's clean search results, without bringing too much functionality.
Google doesn't offer a way to turn off SearchWiki other than logging out from your Google account, but you can install a Greasemonkey script that hides the interface for this additional functionality. If you use Firefox, install Greasemonkey extension, restart the browser and the add the No SearchWiki script. Google's search results will look clean again, even if you are logged in.
Note that the script doesn't disable the SearchWiki interface for the results you've already promoted so you can restore their default position and you can still see the removed results at the bottom of the page. To undo your changes, you can use My SearchWiki Notes page.
Update: You can now disable SearchWiki from Google's preferences page.