Feature: Safari 3 (out of beta!)
How it works: Safari 3 is the third revision of Apple's web browser based on the open source Konqueror HTML rendering engine (also known as KHTML). Apple has open sourced its contributions to KHTML under the WebKit moniker. Safari 3 adds a host of new features to Apple's metallic web browser. Some of my favorites include: enhanced search that makes it easy to see highlighted items, it can remember what tabs you had open and offer to reopen them when you relaunch Safari, integrated PDF tools (so you no longer need to download a PDF and open it in Preview), and it is fast.
Who will use it: Most Leopard users, anyone using an iPod touch or iPhone. It is the default web browser on OS X systems, so that ensures lots of people will be using it (and even our Windows using friends can get in on the fun).