Implementations

The Alcool-90 Experiment
Experimental ML extension with run-time overloading and type-based notion of modules, functors and inheritance. Large HTML text document.
http://pauillac.inria.fr/~rouaix/papers/AlcoolExperiment/plan.html

Dependent ML: DML
Conservative ML extension, has type system to enrich ML with restricted form of dependent types, to allow many interesting program properties: memory safety, termination can be captured in type system and thus be verified while compiling. Description, code samples, downloads.
http://www.cs.bu.edu/~hwxi/DML/DML.html

Extended ML
EML is a framework for specification and formal development of Standard ML programs. Description, publications, tool support, education resources.
http://homepages.inf.ed.ac.uk/dts/eml/

ML Kit
Standard ML compiler.
http://www.it-c.dk/research/mlkit/

MLRISC
Framework for customizable, retargetable and optimizing compiler back ends, written in Standard ML. Successfully retargeted to many architectures. Deals elegantly with special requirements imposed by execution model of different high-level, typed languages.
http://cs1.cs.nyu.edu/leunga/www/MLRISC/Doc/html/

MLton
Whole program optimizing compiler for Standard ML.
http://mlton.org/

Moscow ML
ML system for DOS, Linux, Windows.
http://www.dina.kvl.dk/~sestoft/mosml.html

Open Source Release of MLj
SML to Java bytecode compiler, based on Persimmon's MLj.
http://www.dcs.ed.ac.uk/home/mlj/

Poly/ML
Standard ML full implementation: compiler and environment, has libraries for MS Windows, X Window. [Open Source]
http://www.polyml.org/

Standard ML of New Jersey
SML/NJ is a compiler and programming environment for SML, with associated libraries, tools, documents. Descriptions, news, downloads (Unix, Windows), literature lists, FAQs, links, contacts. [Open Source, Lucent]
http://www.smlnj.org/

TILT
TIL-Two compiler; goal: make compiler for ML-family languages (SML'97, Caml Special Light, KML) that uses types throughout compiling to emit better code without losing safety or correctness; joint effort with Carnegie Mellon. Descriptions, papers, people, links. Cornell.
http://www.cs.cornell.edu/info/people/jgm/tilt.html

TILT Compiler
Self-checking Standard ML compiler, uses Typed Intermediate Languages; while compiling, types are translated with code, type-checking occurs to check compile results; funded by NSF. Descriptions, license, mail list, downloads, links. Carnegie Mellon.
http://www.tilt.cs.cmu.edu/