index.html

<div id="container"></div>
<script src="/socket.io/socket.io.js"></script>
<script>
// <![[CDATA
var socket = io.connect('http://localhost')
, container = document.getElementById("container");

socket.on("message", function (data) {
  container.innerHTML = "<p>" + data + "</p>" + container.innerHTML;
});
// ]]>
</script>

server.js

#! /usr/bin/env node
/**
 * ./server.js tmp.log
 */

"use strict";

var fs = require("fs")
  , child_process = require("child_process")
  , http = require("http")
  , server = http.createServer(handler)
  , io = require("socket.io").listen(server)
  , pid = child_process.fork(__dirname + "/watch.js", process.argv.slice(2));

io.sockets.on("connection", function (socket) {
  console.log("connected");

  pid.on("message", function (m) {
    socket.emit("message", m.added)
  });
});

server.listen(3000);

function handler(req, res) {
  fs.readFile(__dirname + "/index.html", function (err, data) {
    if (err) {
      res.writeHeader(500);
      return res.end();
    }
    res.writeHeader(200);
    res.end(data);
  });
}

watch.js

#! /usr/bin/env node

"use strict";

var fs = require("fs")
  , filename = process.argv[2];

fs.open(filename, "r", "0666", function (err, fd) {
  if (err) {
    console.log(err);
    process.exit(1);
  }

  fs.watchFile(filename, function (curr, prev) {
    if (curr.size <= prev.size) return;

    var buf_size = 1024
      , buf = new Buffer(1024);

    fs.read(fd, buf, 0, buf_size, prev.size, function (err, bytesRead, buffer) {
      var added = buffer.toString("utf-8", 0, bytesRead).trim();
      console.log("" + added + "");
      process.send({added: added});
    });
  });
});