Google have released the long-anticipated official beta version of their Chrome browser for Mac and Linux. The Mac version supports the OS X Keychain and has Mac-style animations; more details here. Meanwhile, over on the Windows version (and the Linux app) there are now new Chrome Extensions which can be used to add functionality to the browser.
There are currently over 300 extensions for Chrome, and details on the Chromium blog as to how developers can create their own. Google promise extensions for Mac soon, though they'll be released through the developer channel and may not be as stable as their Windows and Linux counterparts. You can download Google Chrome for Mac here.