Ubiquity is a Firefox* extension from Mozilla Labs that acts as a "command line for the web" and lets you easily write your own commands and subscribe to commands that others have written.

In this tutorial, I assume you have Ubiquity installed already, and I walk you through creating your first command and sharing it on the Web. The example command is one that looks up members of U.S. congress by zip code.

* - and, eventually, a Thunderbird extension too.