The world of Safari plug-ins has a sparse and somewhat barren landscape; however, sometimes you run into some plug-ins that are must-haves. Keywurl is one such Safari plug-in. Keywurl adds the ability for Safari to do keyword searching just like Firefox does -- which will end up saving you time.
You can assign specific keyword searches in the Safari preferences. So for instance, if I wanted to search for "Steve Jobs" on Wikipedia, then I would type "wiki Steve Jobs" in Safari's address bar; the query would then be sent to Wikipedia, where it would be parsed in a search and the wiki page displayed.
Keywurl is available as freeware from the developer's website. It runs on both Leopard (Mac OS X 10.5) and Tiger (Mac OS X 10.4); and requires Safari 3.0 or later. What's not to love with the price of free and the ability to add custom keyword searches to Safari?