Damn Funny Pictures has small collection of UV tattoos. What better way to let your inner child out than a tattoo with invisible ink? I checked out Wikipedia's entry on the practice, and it includes this warning:
UV tattoo ink is not commonly known or used, as very few varieties of UV inks are approved for use in the US. This is because some people have had reactions to the ink, ranging from minor itching to dermatitis. Several UV inks are suspected carcinogens and allergens and at this time, no research has been conducted into the possible side effects of long term exposure. Some UV inks are known to yellow or turn slightly brown with sun exposure.Apparently this is still a work in progress, and will still likely show up under normal light. For my money, I'd stick with the real thing. Even still, it looks pretty damn cool.
Update: TattooArtists.org has a larger collection of higher resolution photos. (Thanks, Gabriel!)
Link (via Dvorak)