GHC (the Glasgow Haskell Compiler, after its original host university) is the de facto Haskell compiler and simultaneously the main research vehicle for the language and the neighbouring design space in general.

And frankly, while the compiler is awesome and so is the research, the constant churn and seeming inability to settle on what the good programming style and set of features actually is is what eventually turned me away from the language and to the more stable (if near-abandoned) pastures of Standard ML. (That was during the type families upheaval, so, about ten years ago? Don’t know how well it reflects the current state of the project.)