A Compiler Toolkit in Haskell: A set of modules for creating compilers in Haskell. [Open Source, GPL]
FranTk: A high level library for building GUIs in Haskell. It provides good support for developing complex dynamic systems, and is built on top of Tcl/Tk. [Open Source, BSD-like]
Fudgets: Graphical User Interface toolkit for Haskell and X Window system. [Free non-commercial use]
Gtk+Haskell: Bindings for GTK+ for Haskell.
Gtk2Hs: A GUI library for Haskell based on Gtk+. [Open source, LGPL]
Haggis: Multi-threaded, concurrent user interface framework for Haskell, X Window.
HaskellGnome: Haskell Gnome and ORBit effort and bindings
HaSQL Project: An ODBC interface to Haskell implemented using Green Card.
HGL Graphics Library: A portable graphics library for both X11 and Win32. [Open Source, BSD-like]
HOpenGL: An OpenGL/GLUT binding for Haskell. [Open Source, LGPL]
Java VM Bridge: A bridge to the Java virtual machine via JNI for Haskell. [Open Source, LGPL]
MySQL-HS: An interface to the database MySQL for the programming language Haskell. [Open Source, LGPL]
Regular expressions in Haskell: Native Haskell implementation of polymorphic regular expressions. The regexps contains most of the features found in Perl.
TclHaskell: Useful for building graphical user interfaces through the Tcl/Tk. [Open Source, BSD-like]