I have Firefox with any number of search engines (Google, Yahoo, Dogpile, etc.) in the upper right hand corner of the browser. For any of them, highlight the search term you wish to delete and key "shift+delete".
With Firefox, this key removes any search term from any search engine and in addition can remove URL's from your address menu as well.
is the address menu that little thing that autofinishes urls when you type them? because I mispelled drudgereport.com and now the preferred autospelling is a typo-- i'd love to be able to kill that so it autofinishes to the correct address so I don't have to type the whole thing every time.