I regularly search the Drupal API for code. Anyone using Drupal does this. I previously used a Firefox add-on called 'add to search bar' that allowed any search on pretty much any page on the web to be added to the firefox browser toolbar.
However, bukem has created two addons that create this functionality, and do it well. Add to search bar, while it worked, never worked the first time, and had no autocomplete. The two search engines bukem has created both work every time, and also have autocomplete, just to make things a little easier.