Mary Steenburgen is a woman who Jack O'Neill is quite fond of.
In 2000, the artificial intelligence Urgo asked O'Neill "by the way, who is Mary Steenburgen?" after apparently reading her name in his thoughts. (SG1: "Urgo")
In 2004, he told Emmett Bregman "If I could have dinner with anyone in the world, it would be Mary Steenburgen." (SG1: "Heroes, Part 1")
In 2005, in an attempt to show he knew who Jack O'Neill was, Joe Spencer referenced O'Neill's "thing" for Mary Steenburgen. (SG1: "Citizen Joe")
Mary Steenburgen on Wikipedia