Graffiti can be a powerful political force. Before the invention of the printing press and newspapers, graffiti was the tool to influence the masses. Graffiti can spice up fantasy role-playing.
In front of a set of double doors in a dungeon is one of my favorite places to scrawl in dried blood, "Open the other door!" This simple graffiti causes much consternation in the party. Should they heed the warning or not? Should they avoid the doors all together and go another way?
In a previous adventure in the World of Tiglath campaign, the party found some graffiti engraved in the wall. Someone used a stone shape spell to engrave, "Celadus makes the girls moan." The party heard the moans generated by a magic mouth spell. Unseen servant and magic mouth can add interesting dimensions to graffiti. The ruins of Pompeii are the source of the graffiti about Celadus.
"Kilroy was here" is a graffiti from World War II. According to the Straight Dope, numerous newspapers reported pregnant women going to the delivery room with "Kilroy was here" written on their stomachs.
The same Straight Dope article mentions Stalin demanding to know "Who is Kilroy?" after seeing the graffiti in an opulent marble bathroom during the meeting of the Big Three in Potsdam, Germany, in July 1945.
Back in the early 1980's, I dungeon mastered a game each week at the University of Utah. A friend of mine, Harley, created a human fighter named Julian. Harley came up with an interesting list of equipment. Julian, the fighter, always carried several pieces of chalk and paints. Julian left graffiti everywhere he ventured.
I took the graphic from here: http://www.planetcalypsoforum.com/forums/showthread.php?154048-*Free-Event*-Kilroy-Was-Here/page9