Many people noticed a small change: Google has a new favicon. A favicon is a small image that is generally used to reflect the identity of a web site. You can find it in the address bar, in your browser's tabs, in the list of bookmarks or in some feed readers like Bloglines.
Google's favicon is hosted at google.com/favicon.ico and it's a 16x16 pixels image, a standard size for favicons. Google replaced the upper-case "G" in blue border, green and red borders with a lower-case purple "g" in a rounded corner rectangle.
The first time when I saw the new favicon at Google Image Search, I thought there was something wrong with my browser's cache or I typed an incorrect address. Google's new favicon is less cheerful and comforting, but it makes a lot of sense: the small g is a symbol for infinity (∞). A googol (10100) is just a poor approximation for the huge amount of information that needs to be indexed, organized and made useful by Google.
{ Thanks for the tip, Louis, John and Aleksandr. }