Khmer is the official language of Cambodia. Currently the only way to read or type this script correctly on a Mac is to acquire the Xenotypetech Language Kit.
Free Khmer fonts for Windows are available here, but various characters will not display properly in OS X. For a free keyboard layout to use with these, download and install NiDA_Khmer_Mac.1.0.keylayout from this site.
If you have a copy of OpenOffice 2.3/2.4 (the one that used X11, not version 3.0), I think windows Khmer fonts may display correctly in that app.