A smart keyword (also known as bookmark keyword, or keymark for short) is a special kind of bookmark that replaces a special sequence, %s, in the bookmark location field (URL) with the text entered after the keyword in the address bar. For example, if you define "g" for Google Search as shown below:
Pressing Ctrl+L sets the input focus to the address bar and selects the entire content. Then, you enter "g Oracle" in the location bar. After pressing Enter, "g Oracle" will be expanded into:
Ctrl+L > "g Oracle" > Enter
Similarly, Ubiquity command "google" allows you to Google Search "Oracle" in the following keystroke sequence:
Ctrl+Space > "g Oracle" > Enter
assuming command "google" is the top one on the suggestion list. You can read another post for how Ubiquity works here. Both Ubiquity and Smart Keyword allow users to save mouse clicking and word typing to achieve everyday's browser tasks easier and quicker. Then, what's the difference between Ubiquity and Smart Keyword?
In a nutshell, Ubiquity can do what Smart Keyword can do and more. For example, if you select "Oracle" in the browser window, selected word(s) can become the input of Ubiquity command. In this case, you can achieve the same task by:
Selecting "Oracle" > Ctrl+Space > "g " > Enter
Both Ubiquity commands and Smart Keywords function like command templates. Firefox selects one of the commands to execute with command variable replaced with user's input. In Smart Keyword, there is one variable one command. However, you can have more than one variable in a single Ubiquity command.
The centerpiece of Ubiquity architecture is a natural language parser which can provide user a list of candidate commands based on partially matched results. For the top candidate command on the suggestion list, it immediately generates a preview of the top command results in the preview panel. If the top command on the suggestion list is what user wants, user can hit TAB to automatically complete the command name. As user types more characters or uses arrow key to select commands on the suggestion list, Ubiquity dynamically changes the content of suggestion list and command preview.
Besides good for search, Ubiquity commands can be designed to do almost anything. For example, you can translate selected paragraph from one language to another language, insert selected image from search results to the current cursor position, italicize or underline text, etc.
Based on the group activity on email@example.com, you can tell that Ubiquity has active community supports and it evolves fast. This Firefox addon is still in experimental phase (the latest beta is 0.2pre29). There are a lot of design details to be ironed out. But, I think it has potential to become a big hit. At least, I have found it handy and used it on a daily basis.