Open Source Grammar Checkers?

DaveBarr asks: "Maybe I'm more sensitive to this than most, but after continuing to see "it's" instead of "its" and "loose" instead of "lose" everywhere in the media and on web sites of supposedly reputable origin, I began to wonder. Are there any Open Source projects trying to develop a reliable grammar checker -- one that would catch these common foibles? Are all these algorithms proprietary? Are there any University research projects which could be used as a basis for even a halfway-decent grammar checker?"