I am a longtime fan and member of various Rootsweb Mailing Lists. Although these lists are an older IT application they provide a valuable service for family historians. The site states "RootsWeb's mailing lists can help you find information about your ancestors and connect you with people who have research interests similar to yours."
There are presently over 30,000 lists that cover a huge range of interests. One can browse the list of lists via the main categories: surname, USA, international and other, to find one that suits an interest or one can search for a list. Non-listers are also able to search the list archives for content.
I am a member of a number of Australian lists including AUS-NSW-HILLS-HAWKESBURY-HUNTER-VALLEY Mailing List. Through my membership I have made some valuable connections with distant cousins.
For instructions and comprehensive information visit Rootsweb Mailing Lists.
There is no cost associated with the use of these lists.