I came across this free add-in for Apple Safari that lets you customize the we search engine settings. By default Safari only uses Google search. There is no documented way of switching to another search engine.
This add-in is called AcidSearch (get it from here).
You can operate it by clicking the magnifying glass icon on the Safari search box found on the top right of the Safari window. From here you can go to the AcidSearch settings. AcidSearch provides you by default a number of alternate search engines – Yahoo, A9, Vivisimo, AskJeeves .. You can also add your own search engine setting here. From the list here, you can select the search engine you like.
I found that the default url for AskJeeves provided by AcidSearch did not work. This was easily remedied by changing the url to : http://www.ask.com/web?q=.
Overall, this is a great tool that fills in a small void left by Apple.