Critical Mass Modula-3 is an open source implementation of Modula-3 originally created by Critical Mass.
CM3 is the 'main' Modula-3 implementation currently.
There is very little C code in the implementation, even in the runtime.
Recently (About the middle of 2009) CM3 has seen a surge in development, and a stable release is planned. Version 5.8 RC3 is available, with a stable release in the next few months.