Haskell is the most underappreciated yet extraordinarily significant programming language in the world.  The syntax is frightening enough to scare off those with weak stomaches, but some of the most interesting and creative research in type systems and, within recent years, parallelism have arisen from the Haskell community.  I recently stumbled across a fascinating paper from the ACM SIGPLAN History of Programming Languages Conference (HOPL'III) from earlier this year:

A History of Haskell: being lazy with class

Abstract This long (55-page) paper describes the history of Haskell, including its genesis and principles, technical contributions, implementations and tools, and applications and impact.

First I'll admit that I'm a functional programming geek.  Second I'll admit that I love reading about technology history.  But those biases aside, the paper is really quite good.  Recommended reading for anybody who's ever run across a lambda floating around in their dreams.  I know that I have.