Search box bookmarklet for your browser
Bookmarklets are javascript programs that are stored as bookmarks in your browser, and which activate when you click on them. Here's one that conducts a search in Wikipedia on any text you happen to have highlighted in whichever window you are in (or if no text is highlighted, presents you with a popup search box): ~axel/bookmarklet.html. See also the Wikr bookmarklet.
Read more: Bookmarklet