From Belkadan Software:
Safari users: ever wanted to quickly search Wikipedia, or use a search engine besides Google? Then Keystone is what you've been looking for. Type simple queries such as "wiki Mac OS X" directly into the address bar, and Keystone will translate them into searches--in this case, opening the Mac OS X page on Wikipedia. Keystone uses Safari's own completion system, which means you can see all possible searches as you type your query. It also includes autodiscovery, allowing you to turn any search field into a Keystone shortcut.