I know a lot of knitters with knit and spin themed tattoos, but this one is the first I've seen with a practical application.
This is Sophie, who I met at Yarns in the Farms in Beverly Farms, MA, and who has her gauge tattoo featured on YitF's shop blog. It's six inches worth of 1/4 inch marks, which she must find to be very useful while knitting on the subway.
If you're thinking about getting a knitting tattoo, you can also check out the collection that Terry has curated for her brilliant blog, sknitter, and the associated cultural reportage. Considering the possibilities is half the fun.