Linguist is the most important online resource for linguistics, and includes a well-moderated high-volume list with announcements, reviews, etc.
SLLING-L is a discussion list for sign linguistics.
To subscribe, send a message with a blank subject line to listserv@yalevm.cis.yale.edu. In the body of the message type "subscribe SLLING-L Firstname Lastname". To post to the list, send a message to slling-l@yalevm.cis.yale.edu.
All messages to the list are archived online.
Deaf Academics is a web site with related mailing list for Deaf people working in academia. For subscribers, there is also an archive of the list messages.
Mailing list for (sign language) interpreters.