So make sure the designs are of a high enough quality that they will make a good tattoo. Some of the free tattoo designs will end up just being free clip art picked up from some free software program which isn't even intended to be related towards tattooing or tattoos. The disadvantage is that often times the designs will be limited and not so great quality.
The advantage to these sites are that you can access them for free without signing up. There are a number of sites which offer very generic, typical free tattoo designs.