I'm addicted to Firefox's keyboard shortcuts. You can open a new window, enter the address of a website and close the window without moving your hands out of QWERTY position.

The Ctrl-t, Ctrl-k sequence* for opening a search in a new tab is especially useful and, since the release of Firefox 2.0, you can easily add or remove your preferred search engines from the Search Bar drop-down. Simply click the Search Bar drop-down and choose 'Manage Search Engines'. A list of your installed search engines will appear along with an option to get more search engines. However, it's taken me some time to realize that you can switch search engines without moving your hand to the mouse. Smart Keywords are an excellent option, especially when you don't limit them to search engine sites. Unfortunately, not all search engines play well with Smart Keywords. I've found that they are useless for the branched searches on websites like the Merriam-Webster dictionary and Drupal API, for example.

Another option is Ctrl-t, Ctrl-k, Ctrl-(down arrow) or Ctrl-(up arrow). In terms of ergonomic efficiency, this sequence is less desirable than Smart Keywords, since you have to move your hands out of position to reach the arrow keys. Also, you may have to click the down arrow multiple times to find the right search engine. However, I still find the keyboard shortcuts zippier than reaching for the mouse.

* Mac users substitute the command key for the control key