nthChild.js

var nthChild = function (elem, num, fn) {
    var len = elem.length;
    var ret = [];
    var i = 0;

    // :nth-child(num)
    if (!isNaN(Number(num))) {
        for (i = 0; i < len; i++) {
            if (i === num - 1) {
                if (fn) fn(elem[i]);
                return elem[i];
            }
        }
    }

    // :nth-child(numn+num)
    if (num.indexOf('+') > 0) {
        var parts = num.match(/\w/g);
        for (i = parts[2] - 1; i < len; i += parts[0] | 0) {
            if (elem[i]) {
                if (fn) fn(elem[i]);
                ret.push(elem[i]);
            }
        }
    }

    // :nth-child(odd)
    if (num === 'odd') {
        for (i = 0; i < len; i += 2) {
            if (fn) fn(elem[i]);
            ret.push(elem[i]);
        }
    }

    // :nth-child(even)
    if (num === 'even') {
        for (i = 1; i < len; i += 2) {
            if (fn) fn(elem[i]);
            ret.push(elem[i]);
        }
    }

    return ret;
};

usage.js

var rows = document.querySelectorAll('li');

var num = nthChild(rows, 2);
num.className += ' num';

var odd = nthChild(rows, 'odd', function (li) {
    li.className += 'odd';
});

var even = nthChild(rows, 'even', function (li) {
    li.className += ' even';
});

var formula = nthChild(rows, '3n+1', function (li) {
    li.className += ' formula';
});

http://jsfiddle.net/ARTsinn/s3KLz/