Table Information for Current User

I'm trying to create a Client Letter that stamps the name of the user printing the letter in the footer. As the user who prints a letter could differ from both the "Producer" and "CSR" on file for that client, I can't use those fields. Does anyone know in what table I can find the information for "Current User" so that I may enter that as a Merge Field? If it's not in a table, is there another way to stamp a form letter with some identification as to which user printed it? Though preferable, it doesn't have to display a full name, just a unique indentification for internal purposes. Thanks in advance.