Ultraviolet Tattoo.

The ink used in this tattoo is invisible in normal light, but glows in UV light.

Be warned though, it is possible the ink will be slightly noticable if you know it's there, or if minor scarring develops from the tattooing. I'd imagine the effect would be like writing in invisible ink, but I'm not sure.





More here.

Update: Pusa points out that the 'before' and 'after' pictures are of opposite sides of the guy's arms. Here's an 'after' picture from the same side.