This is a very simple Safari 5 extension that allows to select some text in a page and via the right-click contextual menu, submit the search terms to any of the pre-confgured search engines:

The list of search engines is pre-configured but any of the ones available can be toggled on and off, as required:

As of version 1.1, the extension also supports sending the search request to the HTTPS version of the site if available (currently only supported in Google):

The extension can be installed from here: RightClickSearch (right click on the link and then select "Save Link As...", as github.com won't serve the file with the right MIME type)
The code is available from my github repository, here.