Grim Reaper tattoos are not only popular because they look tough, but because they are seen as one of the most detailable tattoos. They are basically a firm variation of skeletal and skulls designs, and they represent the "Death Bringer" itself.
Grim Reapers first appeared on jailbirds and bikers, though they have been adopted by everyday people, to show the embracement of their own mortality. Some people believe that these skeletal figures are evil; some think the Reaper is just a basic symbol of the cycle of life.
The basic design consists of a skeleton with a scythe, in a dark robe. The scythe represents its job of taking souls, and his outlook symbolizes the dead. Sometimes the eyes are shown with fire burning inside, emanating light from the afterlife.
Although Grim Reapers are basically seen as pale or black & white figures, they can be made more exciting by adding colors and other elements. As for colors, red looks good in the image of Death itself - you can add a red robe, a rose, flames, or blood to the design. As for other elements, a graveyard, a gloomy tree looks good in the background. The scythe and hands can be complemented with dices, or more commonly an hourglass.
What's more the bringer of death should not necessarily be a skeleton. Some get designs where a zombie or a demon figure takes the life of the victim.
Then again, Grim Reaper tattoos look good mainly in medium and larger sizes, and when made detailed.