Firefox: Google's clever, mysterious and completely random logo changes, known as "doodles", usually last for just one day. With this Greasemonkey script, you can make any official Google doodle the logo you always see when searching.
The "Favorite Doodle" user script, coded by a Google employee and working only with Greasemonkey on Firefox, links the Google logo on the main and search results pages to Google's gallery of doodles. Under each doodle, the script inserts a "Make This My Favorite Doodle!" button, and clicking it does so. By default, the script will allow new holiday/event doodles to appear, but you can overwrite that behaviour by accessing Favorite Doodle's user script commands from the Greasemonkey menu.
Favorite Doodle is a free download, requires Greasemonkey and Firefox.