I've created a quick reference to "g-code" as implemented in emc2. It's available here, and may be updated from time to time.

A one-page pdf version is also available. This was simply generated by printing the html page in firefox as postscript, then converting to pdf. This file may not be updated when the html version is.

UPDATE: The quick reference card (HTML) version is part of the official documentation. The latest version for emc2.1 is here.
(originally posted on the AXIS blog)