Safari has a search field in the toolbar to make searching convenient, but it is hard-coded to use Google. Glims opens up the search field to a large variety of search engines.
Download Glims here and run the installer. Glims isn't a separate application that you run; rather it is a plug in for Safari. To set Glim's controls, go to Safari's Preferences and choose the Glims tab.
Glims has a huge range of options. Many have been incorporated into Safari4 and many aren't all that useful. What interests me the most about Glims is its Search Engines options. Besides Google, Glims offers about 35 other gnereal esearch engines such as Yahoo! and bing and more specific searches such as Amazon, eBay and Wikipedia.
In Glims' preferences you can uncheck the search engines you don't want to see and drag the search engines up and down the list. If you want to access a search engine not on the list, you can add your own.
Once Glims is set up, you access your search engine selections by clicking on the down arrow at the far left of the search field in Safari's Toolbar. You can change search engines as needed or if you prefer one certain engine you can just set-it-and-forget-it.
As I said earlier, Glims offers many other enhancements to Safari. It might be worthwhile to look through the options and see if any of them interest you. I'd be wary of turning on any options that are already built in to Safari 4 because you might get conflicts. If you turn an option on and don't like how it works, just to back to the Glims tab in Preferences and turn it off.