That's actor Norman Reedus.

"The Bike: A chopped ‘60s Triumph Bonneville TR6C is the bike that Daryl Dixon rides in The Walking Dead. Chopping motorcycles, cutting the frame to make the bike lower, longer and lighter, was a popular amongst motorcycles during the 1960s and 1970s. Daryl’s bike has been obviously customized to be a badass cruiser, complete with a peanut tank. Designed for short, upright, loud rides, this bike definitely fits the image of a badass redneck." Courtesy of http://rideapart.com/2013/01/suspend-your-disbelief-the-walking-dead/