My wife has six; a variety of Greek and classical references (she's a classicist) from her wild youth, and a glyph that I designed for us on her forearm. (Warning: Big picture)
It's two linked Shen rings - Egyptian symbols for eternity and protection, from which cartouches were derived - with the initials "MOW" (Made Of Win) providing a rather pleasing, almost palindromic or ambigrammic symmetry. OK, so we're both geeks. ;-)
Now, if I were to get a tattoo... well, at some point I intend to get that same glyph, when I have the budget and get over my dislike of needles. I'm also tempted to get my old squadron's shield:
And on a more practical note, I've often considered getting important details - name and person number (=social security or NI number, roughly), that sort of thing - tattooed somewhere where they're unlikely to go missing in the event of disaster. Y'know, just to be sure.