linq.js – LINQ for JavaScript, Features:

Implement all .NET 4.0 methods and many extra methods (inspiration from Rx, Achiral, Haskell, Ruby, etc…)
Full IntelliSense support for VisualStudio
Two versions – linq.js and jquery.linq.js (jQuery plugin)
Support Windows Script Host

Source:  linq.js – LINQ for JavaScript.  (http://neue.cc/reference.htm)

http://jsinq.codeplex.com/