Safari 5 added the capability for extensions. Jim Hamm explains, "Here's an article that explains the difference between extensions and plug-ins. You did want to know the difference, didn't you? (grin) Here's a site where you can pick up a few extensions."
He goes on to elaborate, "The availability of many extensions is one reason why Firefox has become such a popular browser. It took a long time for Apple to add this customization to Safari — perhaps because of Apple's penchant to maintain somewhat of a closed, proprietary system."