Today is the first day of summer for many of us, also known as the Summer Solstice. For people in the other hemisphere, like in Argentina, it is the first day of Winter.
Google has logos for both the first day of Winter and Summer. Here they are:
These Google logos were designed by Takashi Murakami, a contemporary Japanese artist.
Yahoo has a nice animated logo:
Bing has a nice theme:
Forum discussion at Google Blogoscoped Forums.