A few highlights:

• Strings

      > "hello world".startsWith("hello")
      true
      > "hi".repeat(3)
      'hihihi'
  

• Object.getOwnPropertyDescriptors() – makes Object.create() more useful.

      var copy = Object.create(Object.getPrototypeOf(orig),
          Object.getOwnPropertyDescriptors(orig));
  
      var newFoo = Object.create(FooProto,
          Object.getOwnPropertyDescriptors({
              instanceProp1: 123,
              instanceProp2: "abc"
          }));
  

• Object.is() – an improved version of === (which will likely become an operator called is in ECMAScript 6).

      > 0 === -0
      true
      > Object.is(0, -0)
      false
      > NaN === NaN
      false
      > Object.is(NaN, NaN)
      true
  

• Map – gives you a dictionary with arbitrary keys.

      > var m = new Map();
      undefined
      > m.set("1", "foo");
      undefined
      > m.set(1, "bar");
      undefined
      > m.get("1")
      'foo'
      > m.get(1)
      'bar'
  

  "1" and 1 are (coerced to) the same key with arrays. Note that each object is considered different from any other object. Hence, the following map entry cannot be easily retrieved:

      > m.set({}, "hello");
      > m.get({})  // new object!
      undefined
  

• Easy install on Node.js (you must run at least Node.js 0.6.5!):

      npm install es6-shim
  

  Afterwards, you enable it in your project like this:

      require("es6-shim");
  

Take a look at the tests to get usage examples.

Source: http://www.2ality.com/2011/12/es6-shim.html