This week, Apple released a preview version of Safari 4. Actually, they released the first version of it a few months back, and have now updated it. It's available as a free download if you have an Apple Developer membership.
There are a variety of new features included in Safari 4, broadly falling into three categories: CSS improvements, HTML 5, and developer tools. Two of the items in the HTML 5 category are of particular interest for building applications that consume APIs or mashup content. These are cross-site XMLHttpRequest and Cross-document Messaging.

