Is there an extension to give Safari the same multiple search engine functionality as Chrome?
Try SafariKeywordSearch. It allows you to set up custom searches triggered by keywords preceding searches. For example, I have it set so that goto Ask Different uses Google's "I'm Feeling Lucky" to send me to the first Google result for "Ask Different".

There's a good set of built-in defaults, and it's pretty easy to add almost any site-specific search out there, by making a small edit to the URL. There are instructions on the site.