There is one I've seen that isn't too obvious. It's Darwin's sketches of the tree of life. If you aren't familiar with them, they simply look like twigs or lines, but if you are, they pretty much spell it out for you.
Or a more contemporary tree of life:
Subtle, very symbolic. I've been considering getting a tattoo of it as well, if I ever get a tattoo.
If you had no other tattoos, and wanted a long one you could get "my other tattoo shows my religious beliefs."
Or you could get Leviticus 19:28 for the irony (the verse forbids tattoos).
If you want something that could be small you could get a symbol like one of these, for example.
I think the IPU symbol has been mentioned by somebody. The symbol I would choose should promote intelligent discussion and this one works for me. Here's a link to some tattoo images of it.
I have the IPU car logo and people are always wondering what it is which gets into some funny conversations where I don't even mention the word 'Atheist'. I think it looks very cool as a tattoo all on it's own regardless of meaning too. Better than all the symbols with the letter 'A' in it somewhere (like a scarlet letter or something).
I know this is a late arrival, but I've been thinking of using Spinoza's Caute! sub rosa. I feel it would be a good reminder that, as atheists, we shouldn't have to hide our beliefs. Of course, I would leave Spinoza's initials out of it. I would consider inserting my own.
SICK BRO ! Actually pretty tight wording too makes sense - i just got ATHEOS going down my tricep
nice, Post a pic!
I am also looking or an appropriate tattoo image that speaks to me. I found this one that I like and am considering getting it on my foot. I like the organic look it has over other typical atheist A symbols.
Hmm I like the electron orbit better in that one, but I also really liked the red A.