To find a good listing of those you can already download, check out the Tumblr site for Safari Extensions. There are already some pretty useful ones such as Gmail This, a way to easily email links to others, and Coda Notes, a way to mark up webpages and show developers what they need to improve visually. If Safari 5 sees anywhere near the amount of activity that Google Chrome has, then expect some pretty powerful extensions in the next few weeks.
It’s good to see that Apple has finally caught up with Firefox and Chrome in this regard. It will surely help their browser market share.