What implementations are available, how do they compare?

All implementations are based on DEC SRC Modula-3. Critical Mass offered an improved version with commercial support. It features incremental garbage collection on NT, and a few additional packages like ODBC database access. This is now open sourced and maintained by Olaf Wagner. Ecole Polytechnique de Montréal has been maintaining an updated distribution. It features integrated documentation, and NT support through the gcc cygwin compiler