Mac's Drive-in in Pendleton, SC was responsible for most of the 40 pounds I gained in college, and I mean that as a compliment. Like Death Valley or Tiger Town Tavern, Mac's is a Clemson tradition that is not to be missed.
The food is typical diner fare, with a variety of burgers and sandwiches. My favorite is the grilled cheeseburger (think grilled cheese sandwich, but with meat). I can't tell you exactly what makes the burgers so good, but I know that I start to get withdrawal symptoms if I wait too long between visits.