APSET: A project to provide better run-time profiling information for GpH.
Buddha: A declarative debugger for the Haskell 98.
DrIFT: A Haskell preprocessor for generating instances of classes.
Green Card: A foreign function interface preprocessor, simplifying the task of interfacing Haskell programs to external libraries.
Happy: A parser generator for Haskell.
HaskellDirect: An (Independent definition language) IDL compiler for Haskell. Can be used to interface foreign language libraries and call COM methods from Haskell.
The Haskell FFI Binding Modules Generator (HSFFIG): A tool that takes a C library include file (.h) and generates Haskell Foreign Functions Interface import declarations for items (functions, structures, etc.) the header defines. [Open source, BSD License]