One of the major features of Safari 5.0 is extensions. Apple promised developers a Safari Extensions Gallery page on Apple.com and recently began accepting submissions. In the meantime, several sites are promoting the growing list of extensions available now.
Another site is SafariExtensions.org. In a worthy attempt to bring order to the world of extensions, the site tries to categorize them. However, there's an awful lot of Misc. in the list.
PimpMySafari.com offers a "pimpified" list of favorites.
The capabilities of the extensions are amazing. For example, Jérôme Gravel-Niquet this weekend released Ostrich, a sweet, free Twitter client that lives in Safari. It's a seamless experience: where- and whenever you're surfing, you have a button to link to that page.
Making Twitter available from every website while you browse it.
Tweet. Retweet. Share links. Get notified of new tweets. Enjoy Twitter more.