Setting your default search provider for Safari on OS X is a simple process: open Preferences in Safari, visit the Search tab and select your provider from the list.
Google, Yahoo, Bing and DuckDuckGo are made available to users.
If you want to search using a different provider, you can either visit the service's dedicated site, change your search provider using the outlined method above, or bypass all of that and use this simple trick.
The next time you enter a search query and want to run it through, say, DuckDuckGo instead of Google, click on the magnifying glass. A drop-down menu will then display the available search options, where you can click on the provider you'd like to use.
You can now search as you normally would, only any future searches would go through the newly selected provider. When you want to switch back (assuming you do), click on the magnifying glass again.
By no means is this a new feature, but it's hidden enough that I'd suspect not many people know it's even there. In fact, I barely discovered it last week.