It’s here, Safari browser extension with the release of Safari 5! Here’s how to start using it:
1. Open Safari Preference, then tick Show Develop menu in menu Bar.
2. From Develop menu, tick on Enable Extensions
4. You can find and manage your installed extensions through Preferences > Extensions
Please also aware that this is still at developer’s experiment access level, so you must prepare for bugs and errors. Apple will open its official Extension Gallery later this summer.