This came to me last night in a dream... and I'm sure I'm not the first guy to think of it but when I woke up I was quite please with myself...
So, simply take the tattoo design of your choice, mirror image it and, using an inkjet printer, print it onto a slick piece of paper (I used the backing of a sticker sheet):
Push that onto your arm and voila... the paper "stamps" the image onto your arm:
Instant tattoo! The ink was still a bit wet in the above photo (hence the shine) but when it dries it's very convincing. Depending on the quality of ink you use (water soluble or not) will determine how long this will last on you.
The applications for this are endless. You could do some quick wounds, scars or other surface detailing quickly and easily.
P.S. The image I used was the one from Mojo's wiki, but colorized to make it a bit more of a dull blue green: http://i251.photobucket.com/albums/g...r-printing.jpg