The Toolbox is a set of navigation-bar items (that column at far right) that can be collectively called tools. These may provide some way to modify the current page, integration with some outside service, or what have you.
Currently, the Toolbox houses the following:
1. A link to automatically generate a printer-friendly version of the page. This is not really necessary, because CSS is used to ensure compliant browsers always send a special printer-friendly version to the printer. However, it is provided as a hedge against noncompliant browsers and for user convenience.
2. A set of links to add the current page to various "Web 2.0" services, such as Digg and Reddit. The main benefit of these links is that they automatically fill in information like the page URL, title, and (for Digg) tags. Descriptions, for sites that use them, are tentatively planned for a future version.