ECMAScript 5 has quite a few nice additions. John Resig has a good overview here. Here is a good ECMAScript 5 compatibility table.
A lot of this stuff can be \"fake
es5-shim http://github.com/kriskowal/es5-shim/
This was part of the narwhal stand-alone javascript environment, but has been broken out on its own. It's pretty darn mature and precise.