The official Linux mascot
It was created by Larry Ewing in TheGIMP, because LinusTorvalds likes Penguins. (After discussions on the linux-kernel MailingList, and an initial suggestion by AlanCox.) Linux.org quotes comments from Linus about how the Penguin was appropriate.
Woodsoup.org has gathered an exhaustive history of Tux. The Google directory has a lot of information about Tux.
There is also a cool game called Tux Racer, where you guide a Penguin down a hill. It used to be Free, but is now Proprietary and commercial. However, see OpenRacer.