This is amusing. If I know programming I would love to crack that wide open to see how it works.
It's not terribly sophisticated. Basically it has two word lists giving a number of relatively subject-neutral words a score proportionally to how often they occur in 'male' versus 'female' writing, adds up their scores for every time they occur, and compares the final scores. For the curious, the entire dictionary it uses is this:
[spoiler] var DictionaryInformal = new Array();
DictionaryInformal['actually']= -49;
DictionaryInformal['am']= -42;
DictionaryInformal['as']= 37;
DictionaryInformal['because']= -55;
DictionaryInformal['but']= -43;
DictionaryInformal['ever']= 21;
DictionaryInformal['everything']= -44;
DictionaryInformal['good']= 31;
DictionaryInformal['has']= -33;
DictionaryInformal['him']= -73;
DictionaryInformal['if']= 25;
DictionaryInformal['in']= 10;
DictionaryInformal['is']= 19;
DictionaryInformal['like']= -43;
DictionaryInformal['more']= -41;
DictionaryInformal['now']= 33;
DictionaryInformal['out']= -39;
DictionaryInformal['since']= -25;
DictionaryInformal['so']= -64;
DictionaryInformal['some']= 58;
DictionaryInformal['something']= 26;
DictionaryInformal['the']= 17;
DictionaryInformal['this']= 44;
DictionaryInformal['too']= -38;
DictionaryInformal['well']= 15;
var DictionaryFormal = new Array();
DictionaryFormal['a']= 6;
DictionaryFormal['above']= 4;
DictionaryFormal['and']= -4;
DictionaryFormal['are']= 28;
DictionaryFormal['around']= 42;
DictionaryFormal['as']= 23;
DictionaryFormal['at']= 6;
DictionaryFormal['be']= -17;
DictionaryFormal['below']= 8;
DictionaryFormal['her']= -9;
DictionaryFormal['hers']= -3;
DictionaryFormal['if']= -47;
DictionaryFormal['is']= 8;
DictionaryFormal['it']= 6;
DictionaryFormal['many']= 6;
DictionaryFormal['me']= -4;
DictionaryFormal['more']= 34;
DictionaryFormal['myself']= -4;
DictionaryFormal['not']= -27;
DictionaryFormal['said']= 5;
DictionaryFormal['she']= -6;
DictionaryFormal['should']= -7;
DictionaryFormal['the']= 7;
DictionaryFormal['these']= 8;
DictionaryFormal['to']= 2;
DictionaryFormal['was']= -1;
DictionaryFormal['we']= -8;
DictionaryFormal['what']= 35;
DictionaryFormal['when']= -17;
DictionaryFormal['where']= -18;
DictionaryFormal['who']= 19;
DictionaryFormal['with']= -52;
DictionaryFormal['your']= -17;
[/spoiler]
(The dictionary is split between formal/informal words as well. Negative values are more 'female.')