Apple released Safari 5  this Monday amidst the announcement of the new iPhone 4G. The update to Safari includes several new features such as Safari Reader, better HTML5 support, and Safari Extensions.
Developers will be able to add their Safari Extensions to a gallery  in which each extension will be signed with a digital certificate. Go here  for a compilation of Safari Extensions currently available until Apple opens their gallery later this summer.
You have to enable Extensions through the Safari develop menu in order to install Safari Extensions right now. Go to the Advanced tab in Safari's preferences and click "Show develop menu in toolbar" if you don't see "Develop" in your menu bar.
Select "Enable Extensions" in the Develop menu once the Develop menu has been enabled. Safari Extensions will be listed in the "Extensions" tab in the Preferences window. From here you can configure Extensions, turn them on and off and enable automatic updates.
Panic, the makers of the excellent web design software Coda , are getting ready to release a Safari Extension called Coda Notes . This extension will allow people to draw, add text and post stickies on websites. These notes can then be emailed them others with a "postcard."
Out of the extensions currently available, my favorite has to be YouTube5 . This extension replaces YouTube videos from flash to HTML5 by converting their tags. One advantage of using this extension is the ability to watch videos with advertisements that can only be played with flash using Youtube HTML5 beta.