In a video demonstration posted after the jump, Mozilla’s Aza Raskin shows among other things how someone inviting a friend to a restaurant could use Ubiquity to find the location map of the restaurant, edit it and insert it in an email. By typing “yelp” and the name of the restaurant, he could add reviews of the restaurant right into the message.
Lots of commands are available by default, but users can also create their own to interact with pretty much any service that offers an open web API. The official website contains examples, a command list, and information about creating your own commands. I really recommend checking it out.