With the introduction of Safari 3, Apple's web browser has seen a significant number of improvements and performance enhancements. The latest nightly webkit builds have again been shown to be significantly faster at Javascript execution than even the final Safari 3.0 release. For comparison, Javascript Speed Test 2007 benchmarks are provided (smaller number faster)

Safari 3.0.4: 445ms
Webkit r28233: 207ms
(Tested on Dual Xeon Mac Pro 2.6GHz)

Webkit is an open source project that serves as the basis for Apple's Safari browser. Webkit's nightly builds offer end-users access to the latest (potentially unstable) Webkit updates. A number of other browsers also use Webkit and will also benefit from these improvements.