There's also an official Twitter extension that looks similar to the Ostrich extension we pointed out yesterday. In fact, extensions are available across several categories, like social networking, news, photos, productivity and security. Root through and find something you'll like; we're partial to Brett's Tablinks extension for copying all the URLs from your open tabs.
Note that you'll need Safari 5.0.1, which was also released today (check Software Update). In addition to the extensions support, Safari 5.0.1 includes more accurate top hits results in the address field and improved scrolling through MobileMe Mail. It's also notable that the Safari Extensions Gallery is now an option in the Safari Menu, just below About Safari. A restart is required to complete installation.
Enjoy the new extensions gallery and point out any gems you find.