Courtesy of Apple
Safari 5 now supports extensions, giving developers and web site builders the ability to add customized browsing tools and features. Safari extensions run in a special 'sandbox' that prevents the extensions from accessing system resources, or being used to gather information on the user. All Safari extensions must carry an Apple-provided digital certificate to ensure the extension is from the source it claims to be from, as well as protect the extension from unauthorized attempts to update it or change its behavior.
Another interesting new feature of Safari 5 is the Safari Reader. When Safari detects that a web page contains a single article, the Safari Reader button becomes available. When you select the button, the page will display the article in larger text, with ads and other clutter removed.
My favorite change in Safari 5 is the return to the progress bar seen in Safari 3 and earlier. The address bar now shows the progress of a page being rendered as a blue bar that fills the address field.
Safari 5 includes 48 security updates, some of which have been around for a while, including a fix for an issue with HTML buttons that could allow hackers to run arbitrary code.
Apple also released Safari 4.1 for Tiger. It includes all of the security updates as well as many of Safari 5's features.
Safari 5 is available from the Software Update item under the Apple menu, or directly from the Apple web site.