Nicole, a physics graduate student and blogger, writes:
My tattoo is the Taylor expansion of sine. I consider it the most beautiful thing I have ever learned (it has held that title for about 7 years) and I got the tattoo after my freshman year at MIT. It has additional meaning to me since sin(x)~x is one of the most useful things in physics.
For a lesson in Taylor series, go here. Click here to go to the full Science Tattoo Emporium.