Just What Is A Mary Sue?S

I know you guys can answer this question for me. I know that the term came into existence to describe idealized (mostly) heroines in Star Trek fanfic, but it seems to have mutated and evolved over the years to encompass a wide variety of characters, both male and female (and in SF and fantasy, presumably non- and ambiguously-gendered). So riddle me this:

  • Of all the protagonists today (genre and otherwise), who best embodies the archetype?
  • Is the term applied fairly?
  • Is Mary Suedom always a negative thing? Aren't there well-written, even popular characters who would fall under the rubric?
  • Who would qualify as an "Anti-Mary Sue"?