Looking for an atheist symbol?  There is no
universally recognized one.  However, the
"Darwin fish," a footed version of the
Christian Ichthys, has become popular, as
has using the word "atheist," or simply "A."

The born atheist fish, created in 2010 by
artist M. Lee, combines the outline of a
Darwin fish, with the word "atheist, and
throws in an "A" (actually an "at" symbol),
for good luck.  Additionally, the fish is
smiling, adding a little good humor to the
battle of the symbols

The born atheist fish is
available for use for
any non-money generating purpose.

Plus, you can get a
free born atheist fish
sticker, for a limited time.  

I think the born atheist fish is the best
atheist symbol available, but there are  
choices ranging from funny to subtle to suit
your personal goals.

More about atheist symbols:

Fishy business

  A's, O's and spelling it out

  Fictitious creatures

  Organizational symbols  
Atheist symbols  (mobile friendly).