The commemorative logo -- called a Google doodle -- is actually an interactive stringed instrument that somewhat resembles a guitar. A visitor can pluck one of the ten strings by either scrolling over the string or using most keys on the keyboard.
The kicker is that the page lets people record and share their recordings. To record, simply click on the button below the doodle. Once you hit the button again to stop recording, you're given a shortened URL to share with others (there are no Facebook or Twitter share buttons, however). Anybody who goes to that URL is taken to a Google search home page and that specific recording automatically plays when the page loads.
Google regularly replaces the official company logo with a doodle - a commemorative version of the logo - at its search page. Over the years, Google doodles have honored the Mars rover Spirit, holidays, the Olympic games and the birthdays of famous people. A full list of Google doodles dating back to 1998 can be viewed here.