gistfile1.js

var CustomEvents = (function() {
  var _map = {};

  return {
    subscribe: function(name, cb) {
      _map[name] || (_map[name] = []);
      _map[name].push(cb);
    },

    notify: function(name, data) {
      if (!_map[name]) {
        return;
      }

      // if you want canceling or anything else, add it in to this cb loop
      _map[name].forEach(function(cb) {
        cb(data);
      });
    }
  }
})();

// in <MyComponent>
CustomEvents.subscribe('foo', function(data) {
  console.log('foo', data);
});

// in <SomeOtherComponent>
CustomEvents.notify('foo', {bar: 7});

In which part of the component lifecycle would I put CustomEvents.subscribe? Anywhere? render?

If using Browserify, Node event emitters is another option: http://nodejs.org/api/events.html

@adelevie

For communication between two components that don't have a parent-child relationship, you can set up your own global event system. Subscribe to events in componentDidMount(), unsubscribe in componentWillUnmount(), and call setState() when you receive an event.