Originally, a non-Jew who does work on Sabbath that a Jew] cannot do. In modern times, it is a non-Jew who toadies to the every wish and whim of the Jews, especially in politics, or a non-Jew who is heavily supportive of Israel.
Rabbi Weisenberg things that Bush is the ultimate shabbos goy. He’s a goyish Menachem Begin, with God on his lips all the time. He's "Good for Jews™"