The Invisible Pink Unicorn: "The Invisible Pink Unicorn (IPU) is the goddess of a parody religion used to satirize theistic beliefs, taking the form of a unicorn that is paradoxically both invisible and pink. This makes her a rhetorical illustration used by atheists and other religious skeptics.
American Atheists Symbol: "The American Atheists logo, based on the atomic model. Officially, the letter 'A' in the middle stands for America, commonly used to refer to the United States. It would, in theory, change for expansion into other countries. The open-ended vertical orbital signifies that not all is known yet." - Wiki
Personally, I like the Flying Spaghetti Monster
This one always gets a chuckle out of me.