I'm surprised no one has mentioned the MoreLINQ project yet. It was started by Jon Skeet and has gained some developers along the way. From the project's page:
LINQ to Objects is missing a few
desirable features.
This project will enhance LINQ to
Objects with extra methods, in a
manner which keeps to the spirit of
LINQ.
Take a look at the Operators Overview wiki page for a list of implemented operators.
It is certainly a good way to learn from some clean and elegant source code.
