Pluralize for Javascript

pluralize.js

Number.prototype.plural = function(){
  if(this > 1 || this == 0){
    return true;
  } else {
    return false;
  }
}

String.prototype.pluralize_rules = function(){
  return [[new RegExp('$', 'gi'), 's']];
}

String.prototype.pluralize = function(number){
  var str = this;
  if(number.plural()){
    var str = this;
    var rules = this.pluralize_rules();
    for(var r=0; r < rules.length; r++){
      if(str.match(rules[r][0])){
        str = str.replace(rules[r][0], rules[r][1]);
      }
    }
  }
  return str.toString();
}

Number.prototype.plural could simply be written as:

Number.prototype.plural = function(){ return this > 1 || !this }

Also, I don't understand why you have to have 2 separate functions which pluralize is calling.
they will never be called outside the scope of pluralize so it's much better to copy those 1-liners inside the pluralize method IMHO.

Also, why did you write var str = this; twice?

I've re-written your code in the way I believe is better:

String.prototype.pluralize = function(count){
  var str = this,
      rules = [[new RegExp('$', 'gi'), 's']], r;

  if( count > 1 || !count ){
    for( r=0; r < rules.length; r++ )
      if( str.match(rules[r][0]) )
        str = str.replace(rules[r][0], rules[r][1]);
  }

  return str.toString();
}

I don't know regex, could you maybe explain what your regex does and why do you need it instead of adding s to the end of the string?
Thanks!