Implementations

Chameleon
Haskell-type language, implements ideas described in a 'A Theory of Overloading'. Compiles Chameleon to (plain) Haskell; can mix Haskell and Chameleon overloading. Descriptions, documents, downloads.
http://taichi.ddns.comp.nus.edu.sg/taichiwiki/ChameleonHomePage

Data Field Haskell
Dialect with instance of data fields (a generalization of arrays), useful for rapid prototyping of parallel algorithms, and parallel high-level system specification.
http://www.mrtc.mdh.se/projects/DFH/

Eden
Parallel functional language to program reactive systems and parallel algorithms using distributed memory. Extends Haskell, but overrules lazy evaluation whenever needed to support parallelism; gives enough control to implement parallel algorithms efficiently, no need for low level details of process management.
http://www.mathematik.uni-marburg.de/~eden/

Glasgow Distributed Haskell
GdH combines the multiple processes of Concurrent Haskell with the multiple processing elements of Glasgow Parallel Haskell (GpH).
http://www.macs.hw.ac.uk/~dsg/gdh/

The Glasgow Haskell Compiler (GHC)
An optimizing compiler for Haskell, written in Haskell. Includes extensions for concurrency and heap and time profiling. [Open Source, BSD-like]
http://www.haskell.org/ghc/

Glasgow Parallel Haskell (GpH)
A Haskell extension for parallel programming built on top of the Glasgow Haskell compiler (GHC). Parallelism is identified using annotations (generally evaluation strategies), that are defined using only two extra primitives: par and seq.
http://www.macs.hw.ac.uk/~dsg/gph/

Gofer
Small interpreter for language based on Haskell report 1.2; experimental, more so for type classes. Haskell adopted some ideas of Gofer, but Gofer type class system is more flexible. Runs on DOS, Linux, Macintosh, Unix. Hugs predecessor.
http://web.cecs.pdx.edu/~mpj/goferarc/

Haskell++
Object-oriented Haskell extension, using a preprocessor, supports a form of inheritance via extended class and instance declarations. Descriptions, documents, source download, links.
http://www.cs.chalmers.se/~rjmh/Software/h++.html

The HBC compiler
A compiler and interactive system for Haskell 1.4, unsupported for two years now.
http://www.cs.chalmers.se/~augustss/hbc/hbc.html

Helium
A functional programming language (subset of the Haskell) and a compiler designed especially for teaching Haskell.
http://www.cs.uu.nl/helium/

Hugs
An interpreter for Haskell which runs on almost any platform. Primarily used for teaching.
http://www.haskell.org/hugs/

Hugs Archive
Many files for Hugs Haskell interpreter: source code, documents.
http://web.cecs.pdx.edu/~mpj/hugsarc/

Hugs98 for .NET
Extension of Hugs98 Haskell interpreter, for interoperation between Haskell and .NET and its framework; can instantiate and use .NET objects in Haskell, and vice versa, call and use Haskell functions from any .NET language.
http://galois.com/~sof/hugs98.net/

Jaskell Project
Java bytecode generator backend for Glasgow Haskell Compiler (GHC), so one can build Java applets or applications via the Haskell language.
http://www.scdi.org/~avernet/projects/jaskell/

Kahu Research
Makes Mondrian, light Haskell version to interoperate with other languages in OO environments; runs on .NET (old version for JVM), supports ASP.NET, to embed functional code in webpages with C#. Has version of Glasgow Haskell Compiler using Mondrian for .NET to compile Haskell for .NET.
http://kahu.zoot.net.nz/

nhc98 from York
A Haskell 98 compiler.
http://www.cs.york.ac.uk/fp/nhc98/

O'Haskell
Object Haskell: Haskell conservatively extended with subtyping, monadic objects.
http://www.cs.chalmers.se/~nordland/ohaskell/

pH (MIT parallel Haskell)
A parallel eagerly evaluated variant of Haskell with syntactic provisions for loops, barriers, and I- and M- structure storage.
http://csg.csail.mit.edu/projects/languages/ph.shtml

PolyP
Polytypic programming language; source files for compiler written in Haskell that compiles PolyP-code to Haskell; library of polytypic functions written in PolyP, some examples, documents, downloads.
http://www.cs.chalmers.se/~patrikj/poly/polyp/

Template Haskell
Haskell 98 extension allowing type-safe compile time metaprogramming, with Haskell, as the manipulating language and the language being manipulated.
http://www.haskell.org/th/