I wrote about the offline feature introduced in Safari 3.1 the other day, and amachang has provided a detailed explanation of how to use it.

What is the "Client-side database storage (SQL API)" implemented in Safari 3.1? - IT War Chronicles

I haven't read it in detail, but it seems to be a specification from HTML5, and is already included in the beta version of Firefox 3.

With this, you can do this and that...