Traditionally, the astrological symbols for Mars and Venus have been used to represent, respectively, male and female. From this, one sees a lot of different uses. Linked pairs of male symbols are often used to represent male homosexuality, and the same goes for linked pairs of female symbols and female homosexuality. Various symbols combining or altering the features of these are often used to represent transgender/transsexual/intersex people (
examples).
It seems to me that there might need to be a symbol for broader genderqueer groups, including those who do not necessarily identify with either a particular gender, nor an intermediate one. While the above article suggests the signs of the Earth and the constellation Sagittarius for anonymous or "unknown" genders, i think another sign might be useful. One possibility is the crescent, the sign of the moon.
Despite what some folks might have you believe, the moon isn't exclusively symbollically female. Even in modern culture, we still speak of the "Man in the Moon"; Mani (Norse), Meness (Latvian), Djehuty (Egyptian), Kalfu (Vodun), Napir (Elamite), Sin (Babylonian), Tsukuyomi (Japanese), and Alignak (Inuit) are all male gods/lwa/kami/spirits associated with the moon or depicted as the moon himself. The Egyptian Khons is traditionally portrayed as a male child, contrary to his usual representations as the patron of the Marvel superhero Moon Knight; Tecciztecatl (Aztec) and the kamui (Ainu) of the moon had both originally been appointed as solar deities, but switched roles with a female deity for various reasons.
The moon is symbollically representative of change, mystery, beauty and majesty. It is associated with silver (with which it which shares the sign of the crescent and the name Luna in alchemy), both are associated with transformation in mythology and folklore.
Other symbols i've seen include butterflies (for transformation), and a flag.
Ray, if you read this, i was wondering what the symbolic meaning of your tattoo is, if anything in particular. Is it related to your gender identity?