**UPDATE: Link fixed! Thanks guys!**
Google is still my search engine of choice, especially now with so many new Google services. But that doesn't mean I've sworn off Yahoo and the other competitors... in fact, I'd rather they were all integrated into one giant search engine for more thorough results, with no brand loyalty required!
That's why I love a handy little Firefox user script called Butler. In a nutshell, Butler will enable you to search any search engine using the Google interface by adding competing search engine links to Google search results. You'll also rid your search results of Google ads, and get around image copyright restrictions on Google Print book results, among a few other tricks.
User scripts are really easy to install and manage (much like a Firefox extension). I didn't pay much attention to their capabilites until Butler came along, but I'm starting to see the light! Please comment if you've got a favorite script you'd like to share.