Ubiquity is a project to develop a linguistic interface for Firefox. It is currently still under heavy development.
The following is a list of commands/verbs I have created for Ubiquity. Since Uniquity's API is still being developed, the commands here are not guaranteed to work. But they should work with the latest code from the repository.
Private Browsing
Easy access to switch between Private Browsing and normal browsing. Requires a Firefox 3.1 nightly build!
Wikipedia
This is now built into Ubiquity!
Search Wikipedia, with article suggestions and preview of summaries and images.
Validate
Validate the markup/css/links of the current page (or any given URL) using W3C's validation services.