In (modern) Greek, there are four cases for nouns and names, depending on their position in the sentence; nominative, accusative, genitive and vocative. Nominative is the subject in a phrase, accusative is the object of the verb, genitive is the possessive case, and vocative is used, for example, to call somebody. "Athanasios" is a formal name, that no one really uses. Instead, the nominative case of my name is "Thanassis" and all other cases are "Thanassi". (Don't generalize this to other greek names, however)
Examples:
"Thanassis authored another nice Slashdot article..."
"Hire Thanassi, and you will get one heck of an engineer."
"The optimal code belongs to Thanassi."
"Hey! Thanassi!"