In principle a open-posting list would be nice, but I don't see
implemented anywhere with reason. My guess would be that no one is
willing to go through literally thousands of SPAM mails every day to
find the one legitimate e-mail coming in once a week that is waiting
for approval by the list admins. Whenever I tried posting as a
non-subscriber to a mailing list my email never went through.
The spam filter is important here. If you try to do it yourself, i
think you are lost. However i'm subscribed to a number of open lists:
five of the ecos-* lists hosted on sourceware.org
lirc on sourceforge
ivtv list on ivtvdriver.org
It is not often i get spam on these lists. I would suggest using one
of the big well known list providers, like sourceware, sourceforge, or
vger and let it do all the hard work.