The CyberSearch FireFox extension transforms your address bar into a dynamic configurable search bar. It searches Google, Wikipedia and more using AJAX. You can customize it fully and even define your own keywords for searching. Like there are few inbuilt keywords, gbook for searching books using Google Books, gloc for searching Local Businesses using Google Local etc. you can even create new keywords from plugin options.
Like I create ap as a keyword for searching in my blog http://anjulsahu.blogspot.com/ [shown in image]. Using cybersearch you can directly open the desired search link through the address bar.
The extension is currently under experimental mode at firefox addon site so you need to register there to install the extension. CyberSearch is free and saves our time of browsing. It should be noted that It works only with Firefox 3.
Here is a video demostrating the plugin.