Today Google had a great logo illustration. The 'l' was replaced with a remembrance poppy. I've often wondered about the Google illustrator. His or her artwork is probably seen by 10s of millions of people on the day however it goes completely uncredited. Anyway Google have an archive for all the previous holiday illustrations. Some are brilliant.
Personally I really like the WaterAid one from March 22nd 2005, partly because I think it's such a fabulous charity. I have a friend who works there who explained to me he chose to join them as they provide the absolute foundation of life. i.e. you can have all the aid flown in that you like, but without sanitation and clean water the community will collapse within days. It's very simple, but pretty compelling really.