3box is a experimental development tool. There is a very brief introductory blog post, and a mailing list.
3box is known to work in Firefox 3+, and seems to work (with limited testing) in Safari 4. It is unlikely to work in other browsers at the moment, though support is planned.
In Firefox 3.5 preview releases and in Safari 4, 3box uses Web Workers, as explained in a second post.
You can create your own 3box with an editable copy of the tutorial which introduces the basic features: