See Add a Tag to DRM-Free Kindle ebooks
for a thread on the earlier teleread article, which suggested drm-free. I think drmfree is ok, we just all need to use the same tag.
I have a python script, kindledrm.py, which can identify DRM-free AZW ebooks, see Kindle Books From Amazon Without DRM
. This really needs to be a self contained GUI program to get widest use.