SML programming language

Standard ML of New Jersey (abbreviated SML/NJ) is a compiler for the Standard ML '97 programming language with associated libraries, tools, and documentation. SML/NJ is free, open source software.

http://cm.bell-labs.com/cm/cs/what/smlnj/index

Moscow ML is a light-weight implementation of Standard ML (SML), a strict functional language widely used in teaching and research. Version 2.00 implements the full SML language, including SML Modules, and much of the SML Basis Library.

http://www.dina.kvl.dk/~sestoft/mosml

Poly/ML is a full implementation of Standard ML available as open-source.

http://www.polyml.org/