This is the nickname for Tom Ricket, a member of the staff of the online comic strip known as Sluggy Freelance.
Shirt Guy Tom is also a recurring character in the comic. He is usually drawn by his namesake, and appears as a crudely rendered stick figure with a t-shirt on. This character is constantly brewing trouble: sabotaging the strip, attempting to assassinate the strip's artist (Pete), being decapitated... things of that nature.
This nickname came about because Tom is in charge of the development and distribution of Sluggy-themed t-shirts. Sluggy-themed shirts are available at: http://store.sluggy.com/ (warning: wearing Sluggy Freelance t-shirts will make you sexy and irresistable.)
In addition to his role on the staff of Sluggy Freelance, Tom was also a programmer on the development team for Diablo II. (For the record, Diablo II is the sequel to the original Diablo, the highly addictive hack-and-slash RPG from Blizzard Entertainment.)