Smart keyword searches are an easy way to use a website's search directly from the Smart Location Bar. To create one, right click the text box that's used for searching, e.g., "search for add-ons" box, and select "Add a keyword for this search". You can then use that keyword followed by the words you would have typed into the text box to search for that directly from the location bar.
This add-on shows those smart keyword searches directly in the suggestions, so you know that you can do a search as well as select it from the list.
Additionally, you can assign multiple keyword searches the same keyword and all of them will show up in the list for you to pick. This is useful for related keywords that you use for the same website.
As an added bonus, if you name the keyword with the same beginning of the site that you're at (e.g., "add" for addons.mozilla.org), when you type "add show keywords", you'll see both keyword searches and normal history results.
Suggestions for popular sites:
add: Firefox Add-ons
def: Answers.com dictionary
map: Google Maps
imdb: Internet Movie Database