On Posting
Before asking a question on a list, first make an effort to find the answer to your question. When you ask your question, (1) be details, and (2) demonstrate that you have made an effort, e.g., "I am having trouble frobbing the foo. I searched the web and only found information regarding how to frob a bar, but that seems unrelated."
List etiquette is to cc the sender and anyone actively involved in the discussion. Some people don't like this and they set their reply-to header appropriately. Respect this.
It is rarely appropriate to cc multiple lists. Sometimes it is. In that case, do so. If in doubt, don't; just choose the single most appropriate list.
When replying to others, please don't top-post and trim the existing text to quote only the sections you're actually replying to. See Posting style for further reading, especially Posting style#Inline replying.
Also see these general notes about communication.
Main Lists
The lists are unmoderated and most of them are hosted on http://lists.gnu.org/. Try to post to the appropriate mailing list.
Some of the Hurd user groups listed on community also host additional mailing lists.
bug-hurd
http://lists.gnu.org/mailman/listinfo/bug-hurd
Technical discussion and bug reports; main development list.
hurd-devel
http://lists.gnu.org/mailman/listinfo/hurd-devel
Low-traffic developers list. This list is moderated and restricted. Subscribe to hurd-devel-readers instead.
hurd-devel-readers
http://lists.gnu.org/mailman/listinfo/hurd-devel-readers
This list is a read-only mirror of hurd-devel. In contrast to subscribing to the latter, everyone is free to subscribe to this read-only list.
help-hurd
http://lists.gnu.org/mailman/listinfo/help-hurd
Hurd-specific questions; for users of the Hurd.
web-hurd
http://lists.gnu.org/mailman/listinfo/web-hurd
Discussion of the http://hurd.gnu.org/ website and this wiki.
l4-hurd
http://lists.gnu.org/mailman/listinfo/l4-hurd
Discussion regarding a possible design successor to the Hurd.
debian-hurd
http://lists.debian.org/debian-hurd/
Discussion around and questions regarding the Debian GNU/Hurd distribution.
gnu-system-discuss
http://lists.gnu.org/mailman/listinfo/gnu-system-discuss
Discussion about the GNU system.
Spam
GNU mailing lists used to have no spam filtering applied -- everyone was allowed to post, the lists were completely unmoderated. This resulted in getting the lists spammed very much, as you can see in the web archives. If you're interested in getting hold of (mostly) spam-free mboxes, then talk to Thomas Schwinge. Also read about the spam-rejection techniques being used today.