Safari 3.1 was just released hours ago on both Macs and PCs, and the new features that come along with it really improve the browser! Download it from here, or simply launch Software Update from the Apple icon in the menu bar.
The most noticeable new feature for most users should be the new ability to double-click on the tab bar to create a new tab, a feature which already exists in most web browsers. Before the update, you could get this functionality through a plug-in called Twicetab.
Other new features in Safari 3.1 are the newly added support for CSS Web Fonts and animations, and video and audio tags in HTML 5, which most (all?) browsers do not yet support.
Gizmodo has a great video of CSS animations in action here.
If the Safari 3.1 update doesn’t seem to be working for you, you should try restarting your computer.