Today I might feel very differently, but I still very much enjoy:
  combinators, tacit programming, the expression problem, trees that grow, extensible records, property-based testing and benchmarking.

cartons was my bump with NixOS.

blood-transfusion readme says it all: overengineered software, meant as an educational endeavor.
  Nevertheless, I recommend a peek.

rose-tree was also important to me, educationally.

hm-lang-light and hm-def-light highlights are the tests - and some weird tacit programming

tsuru-capital-code-sample is ok; but I'd have written it in boring and faster haskell today.

• I used:
  • conduit
  • extensible records with vinyl
  • refinement types with refined
  • money with safe-money
  • etc
• Property-based testing and benchmarking with Quickcheck; see test and bench.