A VERY INCOMPLETE AND UNTESTED JavaScript code rip of Manacher's algorithm. I want to get this into node to actually play with it.

Manacher

/**
 *  Computes the longest palindromic substring in linear time
 *  using Manacher's algorithm.
 *
 *  Credits: The code is lifted from the following excellent reference
 *  http://www.leetcode.com/2011/11/longest-palindromic-substring-part-ii.html
 **/

var Manacher = (function () {
    var palindromic = [],  // p[i] = length of longest palindromic substring of t, centered at i
    	string,  // original string
    	transform,  // transformed string
    	manacher,
    	preprocess,
    	longestPalindromicSubstring,
    	main;

    manacher = function (string) {
        this.string = string;
        transform = preprocess(string);
        palindromic = [transform.length];

        var center = 0,
        	right = 0;
        for (var i = 1; i < transform.length - 1; i++) {
            var mirror = 2 * center - i;

            if (right > i) {
            	palindromic[i] = Math.min(right - i, palindromic[mirror]);
            }
 
            // attempt to expand palindrome centered at i
            while (transform[i + (1 + palindromic[i])] === transform[i - (1 + palindromic[i])]) {
                palindromic[i]++;
            }
 
            // if palindrome centered at i expands past right,
            // adjust center based on expanded palindrome.
            if (i + palindromic[i] > right) {
                center = i;
                right = i + palindromic[i];
            }
        }
    };

    // Transform s into t.
    // For example, if s = "abba", then t = "$#a#b#b#a#@"
    // the # are interleaved to avoid even/odd-length palindromes uniformly
    // $ and @ are prepended and appended to each end to avoid bounds checking
    preprocess = function (s) {
        var t = [s.length * 2 + 3];
        t[0] = '$';
        t[s.length * 2 + 2] = '@';

        for (var i = 0; i < s.length; i++) {
            t[2 * i + 1] = '#';
            t[2 * i + 2] = s.charAt(i);
        }
        t[s.length * 2 + 1] = '#';

        return t;
    };

     // longest palindromic substring
    longestPalindromicSubstring = function (i) {
    	if (!i) {
	        var length = 0,   // length of longest palindromic substring
	        	center = 0;   // center of longest palindromic substring
	        for (var i = 1; i < palindromic.length - 1; i++) {
	            if (palindromic[i] > length) {
	                length = palindromic[i];
	                center = i;
	            }
	        }

	        return string.substring((center - 1 - length) / 2, (center - 1 + length) / 2);
    	} else {
			i = i + 2;
	        var length = palindromic[i],
	        	center = i;

	        return string.substring((center - 1 - length) / 2, (center - 1 + length) / 2);
    	}
    }

     // test client
    main = function(args) {
        var s = args[0],
	        manacher = this.manacher(s);

        console.log(this.longestPalindromicSubstring());
        for (var i = 0; i < 2 * s.length; i++) {
            console.log(i +  ": " + this.longestPalindromicSubstring(i));
        }
    };

	return {
    	manacher: manacher,
    	preprocess: preprocess,
    	longestPalindromicSubstring: longestPalindromicSubstring,
   		init: main
    }
})(Manacher);
var palindrome = "tacocat";
Manacher.init(palindrome);