Use any folder on localhost:3000 (for development)

config.json

{
  "watch":
    { "extensions": ["html", "htm", "js", "css", "less", "json"]
    , "exclude": ["node_modules/", ".git/"] }
, "chrome":
    { "debugging_port": 9222
    , "app_path": "/Applications/Google\\ Chrome.app" }
}

not-quite-the-simplest-express-setup-ever.js

/**
 * Module dependencies.
 */

var express = require('express')
  , http = require('http')
  , fs = require('fs')
  , request = require('request').defaults({ json: true })
  , _ = require('underscore')
  , WebSocket = require('ws')
  , config = require('./config.json');

/**
 * Express setup
 */

var app = express();
config.directory = fs.realpathSync(process.argv[2]);

app.configure(function () {
  app.set('port', process.env.PORT || 3000);
  app.use(express.static(config.directory));
});

http.createServer(app)
    .listen(app.get('port'), function () {

  require('child_process')
    .exec('open http://localhost:' + app.get('port') + ' '
        + config.chrome.app_path + ' --args'
        + ' --new-window --remote-debugging-port=' + config.chrome.debugging_port, function(err) {
      if (err) console.log(err.message);
    });

  walk(config.directory);
  console.log("Express server listening on port " + app.get('port'));
  console.log("Watching directory: %s", config.directory);
});

var refreshBrowser = function(response) {
  var pages = _.chain(response)
    .filter(function(page) {
      return (page.url.indexOf('http://localhost:' + app.get('port')) !== -1);
    })
    .map(function(page) {
      return { id: parseInt(page.webSocketDebuggerUrl.match(/\/page\/([0-9]+)_[12]/)[1])
             , socket: page.webSocketDebuggerUrl.replace('ws://', 'ws://localhost:' + config.chrome.debugging_port)
             , url: page.url } })
    .each(function(page) {
      console.log('Reloading page... [%s] %s', page.id, page.url);
      var ws = new WebSocket(page.socket);
      ws.on('open', function() {
          ws.send(JSON.stringify({ id: page.id
                  , method: 'Page.reload'
                  , params: { ignoreCache: true } })
                  , function(err) { ws.close(); });
      });
    });
};

var watch = function(file) {
  fs.watchFile(file, function(curr, prev) {
    if (curr.mtime > prev.mtime) {
      request.get('http://localhost:' + config.chrome.debugging_port + '/json', function(e, res, body) {
        refreshBrowser(body);
      });
    }
  });
};

var walk = function(dir) {
  fs.readdir(dir, function(err, filelist) {
    _.chain(filelist)
      .map(function(file) { return dir + '/' + file; })
      .filter(function(file) {
        return (_.contains(config.watch.extensions, _.last(file.split('.'))))
            && (!_.any(config.watch.exclusions, function(ex) { file.indexOf(ex) !== -1 }));
      })
      .each(function(file) {
        fs.stat(file, function(err, stat) {
          if (stat && stat.isDirectory()) {
            walk(file);
          } else {
            watch(file);
          }
        });
      });
  });
};

package.json

{
  "name": "dev",
  "version": "0.1.0",
  "main": "index.js",
  "repository": "",
  "author": "[email protected]",
  "dependencies": {
    "express": "~3.0.0rc5",
    "underscore": "~1.4.2",
    "ws": "~0.4.22",
    "request": "~2.11.4"
  }
}

Added features:

1. Target directory can be passed in as an argument.
2. Watches files based on extension w/ the ability to exclude directories.
3. Auto-reloads any open tabs in Chrome that are pointed at localhost:.