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.