Objective Caml

Phantom types: Parameterizing a type with a variable which is unused in the definition of a type. Allows closer alignment of typechecking and properties of the code. See also Generalized Abstract Data Types (GADTs).

Tutorials

How to customize the syntax of OCaml, using Camlp4: Tutorial introduction to O'Caml syntax modification using Camlp4.
Introduction to the Objective Caml Programming Language: by Jason Hickey. PDF.

Flow Caml: The Flow Caml system is a prototype implementation of an information flow analyzer for the Caml language. Its purpose is basically to allow to write "real" programs and to automatically check that they obey some confidentiality or integrity policy. See also Perl's "taint" mode.
MetaOCaml: MetaOCaml is a multi-stage extension of the OCaml programming language. This facility is similar to Scheme and Lisp macros. See also MetaML, 'C (Tick-C).