A super simple module system for browsers. Assumes all source files are concatenated and in browser.

module.js

define.defs = {};
define.modules = {};
function define(name, fn) {
  define.defs[name] = fn;
}
function require(name) {
  if (define.modules.hasOwnProperty(name)) return define.modules[name];
  if (define.defs.hasOwnProperty(name)) {
    var fn = define.defs[name];
    define.defs[name] = function () { throw new Error("Circular Dependency"); };
    return define.modules[name] = fn();
  }
  throw new Error("Module not found: " + name);
}

sampleusage.js

define('foo', function () {
  return 42;
});
define('mymod', function () {
  return {
    stuff: "goes here",
    "and here": require('foo')
  };
});

var MyMod = require('mymod');

I really like this idea.
It's even cooler that node.js added define for defining moduels!
nodejs/node-v0.x-archive@9967c36#commitcomment-458564

With this module in the browser, we can use the same require and define for modules both in node.js and in the browser.