[Nexus]

Сделал "на коленке" минут за 5 (не засекал):

<input type="text" id="input" />
<div id="display"></div>
<script>
    document.getElementById('input').addEventListener('input', function() {
        document.getElementById('display').innerHTML = pack(this.value);
    });

    function pack(str) {
        var last_char,
            counter = 1,
            res = [];

        (str + '.').split('').forEach(function(char, i, list) {
            if (last_char == char)
                counter++;
            else {
                res.push(last_char);
                if (counter > 1)
                    res.push(counter);

                last_char = char;
                counter = 1;
            };
        });

        return res.join('');
    };
</script>