[devnull69]

Your problem is a scope switch. So inside the setInterval loop "this" will no longer refer to your jQuery object but rather to the "window" object. A workaround would be to store the reference and use this temporary variable instead of "this" inside setInterval

(function($){
    $.fn.textAnim = function(text,speed){
        var progress = 0;
        var that = this;
        var a = setInterval(function(){
            progress++;
            $(that).html(text.substring(0,progress));
            if(progress == text.length){
                clearInterval(a);
            }
        },speed);
    };
})(jQuery);