The Arecibo Message:
http://en.wikipedia.org/wiki/Arecibo_message

It came out pretty messy for me, partly because I switched to black too early. I would love to see what it looks like on a better calibrated printer.
Also, it's parametric! You can easily specify the dimensions and whether to make it 3D or 2D for .dxf export.

Print it out and display it somewhere as a reminder that science is cool. Make sure to print in ABS so that it doesn't decompose, so when aliens find it long after our civilization has died out they will be able to learn something about us. ;)