Practically anything in Haskell

* Monads. Yes - the big scary word that makes increadibly easy parsers, IO, operations on Lists and other things so easy (one you notice common pattern)
* Arrows. The same for advanced users ;)
* Standard stuff like lambdas etc. 
* Currying functions
* Algebraic data types
* Pattern matching

And many more.

PS. Yes. I am Haskell fanboy if anyone asked.