Underscore.js

Underscore provides 45-odd functions that support both the usual functional suspects: map, select, invoke — as well as more specialized helpers: function binding, javascript templating, deep equality testing, and so on. It delegates to built-in functions, if present, so JavaScript 1.6 compliant browsers will use the native implementations of forEach, map, filter, every, some and indexOf.

for more details : http://documentcloud.github.com/underscore/