I did not come up with this term, but it is such a useful term in comic critiquing, I think that it is worthwhile to post it.
Here is a good definition that I found of "Mary Sue": MARY SUE (n.): 1. A variety of story, first identified in the fan fiction community, but quickly recognized as occurring elsewhere, in which normal story values are grossly subordinated to inadequately transformed personal wish-fulfillment fantasies, often involving heroic or romantic interactions with the cast of characters of some popular entertainment. 2. A distinctive type of character appearing in these stories who represents an idealized version of the author. 3. A cluster of tendencies and characteristics commonly found in Mary Sue-type stories. 4. A body of literary theory, originally generated by the fanfic community, which has since spread to other fields (f.i., professional SF publishing) because it's so darn useful. The act of committing Mary Sue-ism is sometimes referred to as "self-insertion." Warren Ellis is considered to be a prominent user of Mary Sues (Pete Wisdom in Excalibur, for one).
Am I missing any other significant, non-Ellis Mary Sues?